For basic information on the disease and how individuals can mitigate the spread, the following resources provide excellent information:

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1. COVID-19 TESTING: THE BASICS

  • What's the difference between PCR and antigen tests?

     

    PCR tests, also known as molecular tests, look for pieces of the virus’s genetic material using a lab technique called polymerase chain reaction (PCR). They are the most accurate kind of test for COVID-19 currently available.

    PCR samples are usually sent for processing and analysis in a lab, and results can take anywhere from a few hours to several days to arrive. Lab-based PCR tests tend to be more costly than other types of test. Some point-of-care PCR tests are now available, but they typically require purchase of a dedicated instrument and training to run the tests. If you want to use a point-of-care PCR test to screen members of an organization, you may need to purchase multiple instruments and train several staff members for this purpose. Point-of-care PCR tests typically return results in under an hour.

    Antigen tests look for viral proteins. These tests typically provide results within minutes and are very good at accurately identifying people who are shedding a lot of virus particles. They are not as good as PCR tests are at finding people who aren't carrying as much virus, including children and asymptomatic adults. If you are using this type of test in an organizational setting, be sure to understand how well the particular brand you use works in the population you’ll be testing (adults vs. children, people with symptoms vs. people without symptoms). Many antigen tests still require trained staff to run them, but a few are now available over the counter. Antigen tests tend to be the least expensive type of test.

    Because of their decreased sensitivity, antigen tests have the following limitations:

    • If you are using antigen tests to screen members of an organization, you may need to test more frequently than you would if you were using a PCR test in order to prevent an outbreak.
    • Positive results on antigen tests whose specificity is >99% are usually correct, but negative results may need to be confirmed with a PCR test.

    Remember, no test is perfect. 

    Coronavirus Disease 2019 Testing Basics

    CDC - Interim Guidance for Rapid Antigen Testing

  • What is the difference between point of care (POC) testing and lab-based testing?

     

    In point-of-care (POC) testing, all testing steps, including sampling and analysis, take place close to or near the patient. For the When to Test calculators, this means that testing is completed either on-site, at your organization’s location (for the Organizational Calculator) or at a testing site, in a health-care provider's office, or in your home (for the Individual Calculator). Point-of-care tests give you results quickly. Most of them are antigen tests, but some point-of-care PCR tests are available, as well.

    In lab-based or off-site testing, samples are gathered on-site at your organization or other testing location and are sent to a central lab for processing and analysis.Lab-based tests are generally PCR tests, which are highly accurate. However, they take longer to return results than point-of-care tests do. In some cases, the turnaround time of lab-based tests is not fast enough to prevent an outbreak in an organization.

    CDC - Guidance for COVID-19 Point-of-Care Testing

  • How do COVID-19 variants affect testing?

     

    The CDC and FDA, in partnership with the major testing laboratories, continue to carefully monitor the emergence of variants and how their genetic characteristics might affect testing.

    NOTE: As part of its calculations, the When to Test Calculator includes a default measurement of how transmissible the virus is. That measurement is called R(“R-naught”) or the basic reproduction number for the virus. WhentoTest.org follows CDC guidance to set the Calculator’s default for R0. If you would like to set a specific R0 in your scenarios, you can do so under Advanced Settings: Main Calculator Settings.

    CDC COVID-19 Pandemic Planning Scenarios

    FDA Issues Alert Regarding SARS-CoV-2 Viral Mutation

    CDC COVID-19 Pandemic Planning Scenarios

    FDA Issues Alert Regarding SARS-CoV-2 Viral Mutation

  • What's the difference between a Rapid Antigen Test and Antigen w/ Instrument in the Calculator?

     

     

    Both Rapid Antigen tests and Antigen w/ Instrument tests are point-of-care COVID-19 tests that look for viral proteins and return results within minutes. For the purposes of the When to Test Calculator, the differences between these two types are as follows:

    Rapid Antigen Test

    • Uses paper-based lateral flow technology
    • Looks like a pregnancy test
    • Once the test is used, it is discarded
    • Some of these tests can be completed by the person being tested; others require trained staff to obtain samples and/or run the test itself
    • Has a lower sensitivity than Antigen w/ Instrument (see note below)

    Antigen w/ Instrument

    • Requires a small piece of equipment to run the test
    • The instrument must be purchased separately from test kits and maintained over time
    • Your organization may need to purchase several instruments in order to be able to test your population quickly enough
    • Requires trained staff to run the tests and maintain the instrument(s)
    • Has a higher sensitivity than Rapid Antigen (see note below)

    IMPORTANT NOTE: The sensitivity and specificity of both rapid antigen tests and antigen with instrument tests vary a great deal between test brands. If your organization decides to use one of these test types either for asymptomatic screening or for follow-up for pooled testing, check the sensitivity and specificity of the brand of test you plan to use. Then go to Advanced Settings in the When to Test Calculator, click on the tab for the relevant test type, and adjust the sensitivity and specificity settings to match your brand of test.

  • What is follow-up testing, and how does it work?

     

    Follow-up testing is a critical part of pooled testing. In pooled testing, samples are gathered from multiple people and mixed together into a pool. Instead of testing each individual sample on its own, the lab tests the pool. If a pool tests positive, then at least one individual in the pool may be positive for COVID-19. When this happens, follow-up testing, sometimes referred to as “reflex testing” or “deconvolution,” is required. If follow-up testing isn’t possible, then the entire pool should isolate.

    In follow-up testing, each person or each sample in the positive pool is individually retested to determine which individual(s) in the positive pool are infected. The way follow-up testing happens depends in part on how and where the samples are pooled.

    The When to Test Calculator identifies four testing strategies for follow-up testing a positive pool. Details on each strategy appear beneath the table below.

    Automatic

    Pooling Process: Individual samples are collected on-site and taken to an off-site lab. The lab creates the pools and tests the pooled samples using PCR.

    Follow-Up Testing Process: When a positive pool is identified, the lab automatically retests the individual samples that went into the pool using PCR.

    Pros and Cons

    • Eliminates need to re-collect individual samples.
    • Results are provided faster than other pooled testing strategies.
    • May be 2-3x more costly per test than rapid antigen and on-site PCR follow-up.

    On-Site Rapid Antigen

    Pooling Process: Individual samples are collected and pooled on-site. The pooled samples are taken to an off-site lab, which tests them using PCR.

    Follow-Up Testing Process: When a positive pool is identified, all individuals in the positive pool must be resampled. Individual follow-up tests are completed on-site, using rapid antigen tests.

    Pros and Cons

    • Faster than off-site PCR follow-up.
    • Likely to be least expensive option.
    • Requires trained staff on site.
    • Slightly less accurate than PCR follow-up.

    On-Site PCR

    Pooling Process: Individual samples are collected and pooled on-site. The pooled samples are taken to an off-site lab, which tests them using PCR.

    Follow-Up Testing Process: When a positive pool is identified, all individuals in the positive pool must be resampled. Individual follow-up tests are completed on-site, using PCR instruments.

    Pros and Cons

    • Faster than off-site PCR follow-up.
    • Less expensive than automatic or off-site PCR follow-up.
    • Requires PCR instruments and trained staff on site.
    • Highly accurate.

    Off-Site PCR

    Pooling Process: Individual samples are collected and pooled on-site. The pooled samples are taken to an off-site lab, which tests them using PCR.

    Follow-Up Testing Process: When a positive pool is identified, all individuals in the positive pool must be resampled. Samples are shipped to an off-site lab, where individual follow-up tests are completed with PCR.

    Pros and Cons

    • Slowest pooling option.
    • More expensive than on-site follow-up.
    • Highly accurate.

    For more information on pooled testing, please download our K-12 Playbook where you can read more about pooling on page 20.

  • What’s the difference between COVID-19 screening (aka asymptomatic screening) and diagnostic testing?

     

    Screening for COVID-19, also referred to as screening testing or asymptomatic screening, means testing your population to identify infected people who are not showing symptoms. Diagnostic testing for COVID-19 is performed when infection is suspected, such as when the person:

    • Has COVID-19 symptoms or
    • Has no symptoms, but has had a recent known or suspected exposure to SARS-CoV-2 (the virus that causes COVID-19).

    Asymptomatic screening is typically a regularly scheduled event (e.g., 1x/week). Along with other COVID-19 mitigation strategies, screening can help prevent outbreaks in your school or workplace.

    Not many tests have authorization from the FDA specifically for screening testing. However, the FDA supports the use of COVID-19 tests that have Emergency Use Authorization (EUA) when used “off-label” under the supervision of a physician or other prescriber.

    NOTE: The When to Test Calculator assumes that tests that do not have FDA authorization for screening are used off-label under practitioner guidance.
     

  • I’m worried that if I test asymptomatic people, I’ll get a lot of false positives and will have to isolate and quarantine people unnecessarily. What do I do?

     

     

    The best way to avoid false positives is to choose a test with high specificity (>99%). A test’s specificity indicates how good it is at designating someone who DOESN’T have the disease as negative. People who test positive using tests with high specificity are very likely to truly have the disease, which means that very few people will be isolated and quarantined unnecessarily.

    If you’re in an area with very low prevalence of disease, it’s especially important to choose a test with high specificity, because the likelihood of false positives becomes higher under those conditions.

  • I'm not familiar with on-site PCR tests. How are they different from antigen tests and in-lab PCR tests?

     

    On-site PCR tests use a small instrument to conduct a polymerase chain reaction (PCR)-based test on a sample. Depending on the size of your organization, you may need to purchase multiple instruments in order to test your population quickly enough to prevent an outbreak.

    On-site PCR tests are not quite as sensitive as in-lab PCR tests, but they are typically more sensitive than antigen tests. These tests return results quickly – typically in under an hour – but are not as fast as rapid antigen tests. They require trained staff to run and maintain them.

  • How can I tell if pooled testing is a good fit for my organization?

     

    Pooled testing, also known as pooling, is a way to provide highly sensitive PCR testing for your organization in a more cost-effective way. In pooled testing, samples are gathered from multiple people and mixed together into a pool. Instead of testing each individual sample on its own, the lab tests the pool, thus using fewer resources and lowering processing costs.

    Pooled testing could be a good fit for your organization if:

    • Positivity rates in your community are relatively low (i.e., you don’t expect a lot of people in your organization to test positive)
    • You want the sensitivity of PCR testing
    • You don’t want to train or pay staff to do testing on-site (however, some types of pooled testing do require on-site follow-up testing)
    • You can’t test more frequently than once a week

    Pooled testing is not a good fit for your organization if:

    • Positivity rates in your community are very high (i.e., you expect a lot of people in your organization to test positive)
    • You need immediate test results

    CDC – Guidance for Use of Pooling Procedures

    FDA – Pooled Sample Testing

    Open and Safe Schools — Includes toolkit on how to start a pooled testing program at your school.

  • There are a lot of different kinds of antigen tests out there. How do I select one for my organization to use?

     

    In addition to the cost of the individual test kits, the following criteria can inform your choice:

    Accuracy: Most antigen tests have a very high specificity. However, the sensitivity of the different brands of tests varies. The When to Test Calculator assumes a lower sensitivity for Antigen tests than with PCR tests, to ensure that the recommended testing frequency is adequate to prevent an outbreak.

    Capital expenditures and maintenance: Some antigen tests run on a small testing instrument, which must be purchased separately and maintained over time. Depending on size of your organization, you may need to purchase several of these instruments in order to be able to test your population quickly enough to prevent an outbreak.

    Staffing requirements: Some antigen tests require trained staff to obtain samples and/or run the tests. If you use an instrument-based antigen test, you will need staff to maintain the instruments, as well. Other types of antigen tests can be performed entirely by the person being tested.

    Time: Some rapid antigen tests can be done at home, decreasing the amount of time taken away from work or school for testing purposes.

    Access: Manufacturing of antigen tests has not always kept up well with demand, so some businesses may find they have a limited selection of brands from which to purchase. Some schools will only have access to a single type of antigen test, which is provided or mandated by the state.

  • What is pooled testing?

     

    Pooled testing, also known as pooling, is a way to make highly sensitive PCR testing more cost-effective. In pooled testing, samples are gathered from multiple people and mixed together into a pool. Instead of testing each individual sample on its own, the lab tests the pool, thus using fewer resources and lowering processing costs.

    If a pool tests negative, then all individuals in that pool are ‘clear’ or negative for COVID-19 and may continue to attend class, work, or other activities.

    If a pool tests positive, then at least one individual in the pool may be positive for COVID-19. When this happens, follow-up testing, sometimes referred to as “reflex testing” or “deconvolution,” is required. Follow-up testing can be done at a central lab or on-site.

    In follow-up testing, each person or each sample in the positive pool is individually retested to determine which individual(s) in the positive pool are infected. If follow-up testing isn’t possible, then the entire pool should isolate. Pooled testing should be done at a lab, using a PCR test that has Emergency Use Authorization (EUA) from the FDA. The lab’s pooling system should either have EUA or be internally validated.

    CDC – Guidance for Use of Pooling Procedures

    FDA – Pooled Sample Testing

    Open and Safe Schools — Includes toolkit on how to start a pooled testing program at your school.

    When To Test K-12 Playbook for COVID-19

  • What kinds of tests can be used for COVID-19 screening or diagnostic testing?

     

    Both kinds of testing can be done using either a PCR (also known as molecular) or antigen test.

    PCR tests look for pieces of the virus’s genetic material. They are the most accurate kind of test for COVID-19 currently available.

    Antigen tests look for viral proteins. They are slightly less accurate than PCR tests.

    Antibody tests (also known as serology tests) look for antibodies to SARS-CoV-2 (the virus that causes COVID-19). Those antibodies can come from a past COVID-19 infection or from a COVID-19 vaccine. This kind of test cannot be used to diagnose an active COVID-19 infection.

    CDC - Interim Guidance for Rapid Antigen Testing

  • What is confirmatory testing, and why would my organization need it?

     

    Confirmatory testing is done to make sure the results of a test are correct. In most cases, confirmatory testing is used to verify whether a negative result on a COVID-19 antigen test is accurate.

    If a test with a specificity of less than 99% comes back positive, especially in low-prevalence environments, confirmatory testing is also recommended. Asymptomatic screening programs are frequently supplemented by confirmatory testing because false positives are more common when screening asymptomatic individuals.

    CDC - Interim Guidance for Rapid Antigen Testing

  • What is an antibody test? Can it be used to diagnose a COVID-19 infection?

    Antibodies are proteins created by your immune system to help you fight off infectious microbes. Your body makes them after you’ve been infected or vaccinated.

    Antibody tests (also known as serology tests) look for antibodies to SARS-CoV-2 (the virus that causes COVID-19). They cannot be used to diagnose an active COVID-19 infection, because it can take up to three weeks after the infection for your immune system to make antibodies. These tests can only be used to determine whether someone had COVID-19 or a COVID-19 vaccine in the past.

  • What are sensitivity and specificity?

     

    Sensitivity refers to how well a test designates someone who HAS the disease as positive.

    If you have a test with 95% sensitivity and you test 100 people who have COVID-19, 95 will test positive (true positive) and 5 will test negative (false negative). Tests with high sensitivity will find most cases of the virus. They will return few false negatives.

    Specificity is how well a test designates someone who DOESN’T have the disease as negative.

    If you have a test with 95% specificity and you test 100 people without COVID-19, 95 will test negative (true negative) and 5 will test positive (false positive). People who test positive using tests with high specificity are very likely to truly have the disease. These tests return few false positives.

    In an ideal world, all tests would be both highly sensitive and highly specific. Unfortunately, that’s not possible most of the time. It’s also important to acknowledge that no test is perfect - no test will have 100% sensitivity and specificity.

  • What does Emergency Use Authorization (EUA) mean?

     

    In the US, the use of medical tests is regulated by the Food and Drug Administration (FDA). Under normal circumstances, these tests can only be used in this country once they’ve undergone a months-long approval process. In early 2020, the US was in desperate need of tests for COVID-19; we couldn’t wait months for tests to arrive. For this reason, the US Secretary of Health and Human Services declared that the FDA could use Emergency Use Authorization to make COVID-19 tests available more quickly.

    As its name suggests, EUA is a seldom-used type of authorization which is only allowed during public-health emergencies. It enables the FDA to authorize the use of medical products (including tests) that haven’t undergone the long approval process, provided that “certain criteria are met.” At a minimum, the product must have known and potential benefits that outweigh its known potential risks. In addition, there must be “no adequate, approved, and available alternatives” to the product receiving the EUA.

    There are currently no COVID-19 tests that have complete approval from the FDA. Those that have EUA have undergone a shortened version of the approval process, which ensures that they meet the criteria mentioned above, among other requirements. The When to Test Calculator assumes that your organization is using a test that has received EUA.

    FDA – Emergency Use Authorization
     

2. COVID-19 MITIGATION STRATEGIES

  • How does limiting the number of unmasked people who are allowed to be in close proximity indoors or to participate in high-COVID-risk activities together indoors affect my organization's testing needs?

     

    Limiting the size of groups who participate in these kinds of activities together indoors and unmasked is an extremely effective way to reduce COVID-19 transmission risk, and thus reduce your organization’s need for testing.

    With increasing levels of vaccination in the US, many organizations have decided to do away with mask-wearing altogether. However, some activities remain higher risk than others in terms of their potential for COVID transmission. Any activity that brings a group of people together in close proximity for an extended period of time – most commonly, eating and/or drinking together at the same table – carries a high risk. Other high-COVID-risk activities are those that involve breathing heavily and/or with force, such as sports activities, singing, and playing wind instruments, whether or not the participants are in close proximity to one another.

    If your organization permits either of these types of activities indoors, then the number you would input in this section of the Calculator is the largest group that participates in such activities together. For example, if you have a choir of 10 people and a basketball team of 25, you would input “25.” Doing so allows the Calculator to factor in the additional layer of risk added by permitting these kinds of activities to happen indoors.

    Note - The Calculator assumes that cohorts of people eating or drinking together are physically distanced from other people in the room. Thus, if your cafeteria has 20 tables that each seat a cohort of eight people and the tables are more than six feet apart, the size of the cohort is eight.

    However, physical distancing has its limits. If people in your organization are participating in unmasked indoor group activities that involve significant exertion or air expulsion (for example, sports, gym classes, or singing), then the cohort size should be the total number of people in the room, regardless of their distance from one another.

    CDC - Resource on Cohorts in a K-12 school

    Oregon Department of Education – Cohorting  One example of a school-based cohorting system.

  • How does mask wearing impact my organization's need for testing?

     

    The more people in your organization who wear masks consistently and properly, the less testing you will need to do to prevent an outbreak. Remember: a well-fitted mask, worn properly, covers both the nose and the mouth and does not leave gaps.

    The When to Test Calculator asks you to estimate what percentage of people in your organization wear their masks properly at all times when indoors. Most people tend to overestimate the percentage of people in their own organizations who do this, so you may want to input a number a bit lower than you were first considering.

    The Calculator assumes that most people in your organization wear cloth masks, which have a fitted filtration efficiency of approximately 30% (0.3). If your organization requires that everyone wear N95 masks (approximately 95% efficiency) or KN95 masks (approximately 85% efficiency) when indoors, that will significantly change your testing requirements, and you should adjust the Calculator’s settings accordingly. To do that, click on the Show Advanced Settings button on the results page, open the Main Calculator Settings tab, and scroll down to the Mask Efficiency setting (green arrow below).

    Find high-quality masks at Project N95.

    CDC - Use Masks to Slow the Spread of COVID-19

  • How does my organization's ability to identify and notify close contacts affect our testing needs?

     

    The faster your organization can identify and notify close contacts of an infected person, the lower your need for testing will be. The reason for this has to do with how contact tracing works and how it affects the likelihood of an outbreak.

    The goal of contact tracing is to identify the people who were in close contact with an infected person while they were contagious, so that those people can be prevented from spreading the disease to others. The faster close contacts are identified and notified of their status, the sooner they can quarantine, and the less likely it is that they will infect other people.

    For COVID-19, the definition of a close contact is someone who was within six feet of an infected person while they were contagious, for a total of at least 15 minutes over the course of 24 hours. When someone with no symptoms tests positive for COVID-19, it’s assumed that they were contagious for the 48 hours (two days) before they provided the sample that was tested.

    In practice, identifying exactly who has been in “close contact” with someone over the course of two days can be quite difficult unless your organization keeps good records. Schools often keep seating charts for classrooms, school buses, and lunch tables for this purpose. In addition, teachers may keep track of which students tend to walk next to one another in the hallways. In an office setting, keeping records of in-person meeting attendance and assembly-line stations may be helpful.

    NOTE: The When to Test Calculator assumes that only 50% of an infected person’s close contacts are identified and notified of their status within 24 hours (a contact-tracing efficiency of 0.5). This assumption may be changed in the Calculator’s defaults under Advanced Settings: Main Calculator Settings.

    CDC - Close Contact definition

    CDC - Contact Tracing for COVID-19

    CDC – What you can expect to happen during contact tracing

  • Can my organization require that our population (i.e., employees and/or students) be vaccinated?

     

     

    For information about this and other questions about COVID-19 regulations in the workplace, please check the US Equal Employment Opportunity Commission’s (EEOC) website, listed below.

    Vaccination requirements for students are typically set at the state level. Please contact your state department of education for further guidance.

    EEOC – Coronavirus and COVID-19

    US Department of Education – State Contacts
     

  • Can my organization ask members of our population (i.e., employees and/or students) whether they have been vaccinated?

     

    Yes, but people may choose on an individual basis to decline to answer the question. Once an employer has information about an individual’s vaccine status, they must keep it confidential and separate from the rest of the employee’s personnel files, per the Americans with Disabilities Act (ADA).

    US Department of Health and Human Services – The HIPAA Privacy Rule

    EEOC – Coronavirus and COVID-19

  • How does vaccination impact my organization's need for testing? Do vaccinated people still need to be tested?

     

    The more people in your organization who are vaccinated, the less testing you will need to do to prevent an outbreak. If you aren’t sure what percentage of your organization has been fully vaccinated, you can use published estimates for your area, available via the links below.

    While the CDC recommends that fully vaccinated people “refrain from routine screening testing if feasible,” participation in screening testing is contractually required in some workplaces. Thus, the When to Test Calculator assumes that vaccinated individuals in your organization will continue to participate in screening. If you plan to test only the unvaccinated members of your organization, then you should model them as a separate group.

    CDC – Interim Public Health Recommendations for Fully Vaccinated People

    CDC – COVID-19 Integrated County View — Does not include Texas, Hawaii, and some counties in California and Alaska.

    CDC – COVID-19 Vaccinations in the United States — State-by-state vaccination statistics.
     

3. USING THE WHEN TO TEST CALCULATOR

  • The Calculator is telling me "Turnaround Time Too Long." What does that mean?

     

    Testing can only help prevent an outbreak if your organization receives test results in time to isolate infected people and quarantine their close contacts before they can spread the virus. In some situations, the amount of time it takes Off-Site PCR and Pooled PCR results to come back will be too long, based on the Calculator's default settings.


    If turnaround time is your problem, then you may be able to work with your lab to fix it. The Calculator assumes that it takes 36 hours to receive test results from an off-site lab. If your lab can return results in less time, go to Advanced Settings and click on the tab for the type of test you've chosen. Then scroll down to "Lab turnaround time, in hours" (green arrow below) and change the default there.


    And remember, even if the Calculator is giving you this result, improving your mitigation strategies is likely to help you to decrease the likelihood of an outbreak, as well.

  • How do I know if I need to test at my organization at all? When can I stop testing?

     

    If your conditions are favorable and your organization has excellent mitigation measures in place, you may not need to regularly test people without COVID-19 symptoms in order to effectively reduce the chance of an outbreak. In those cases, the Calculator will recommend “Symptomatic Screening Only,” meaning that you should continue to require COVID-19 testing for people who display symptoms, but you do not need to test those who are symptom-free.

    To reach this goal, consider how your organization might improve mitigation measures other than testing, such as by emphasizing consistent mask-wearing, incentivizing vaccination among employees, keeping better track of close contacts, or decreasing the size of cohorts that participate in unmasked activity together. Use the Calculator to model various scenarios and predict how improvements in these areas can change your test recommendations.

    Once you’ve modeled a scenario that you believe is achievable, take steps to make those improvements a reality within your environment.

    NOTE: The Calculator’s results for typical conditions are based upon the CDC’s guidance of R0, and upon the assumption that your organization has already performed baseline testing (testing 100% of all of its members prior to beginning an asymptomatic screening program). It’s a good idea to be prepared to institute asymptomatic screening should your organization’s situation change, and to frequently recheck the Calculator’s results for your organization.

  • I thought my organization's mitigation strategies were adequate, but the Calculator gives me a warning message when I input those strategies. Why is this happening?

     

    There are three main reasons why you might get that result.

    Regardless of the reason why you received the warning message, improving mitigation strategies (incentivizing vaccination among employees, emphasizing consistent mask-wearing, keeping better track of close contacts, and/or decreasing the size of cohorts that participate in unmasked activity together) is the best way to lower your organization's risk of an outbreak.

  • The Calculator is telling me "Test Days Inadequate." What does that mean?

     

    The Calculator assumes that people in your organization are on-site and can be tested five days a week. If you get this result, it means that people would need to be available for testing more than five days a week in order to reduce the chance of an outbreak. (Note: For the purposes of this result, the Calculator also assumes that testing is spread out over the course of a week, and different individuals in your organization get tested on different days.)

    If testing takes place at your organization seven days a week, go to Advanced Settings, click on the Main Calculator Settings tab, and change the setting on "Days per week that testing is done?" to "7."

    Note that in some situations, even daily testing will be inadequate; in those cases, you may need to improve your mitigation strategies in order to decrease the risk of an outbreak. And remember, even if switching to daily testing does solve the problem, improving your mitigation strategies may help outbreaks become even less likely.

  • The Calculator is telling me "Mitigations Inadequate." What does that mean?

     

    If you see this result, you will need to improve your organization's COVID-19 mitigation measures other than testing in order to help decrease the risk of an outbreak; adding testing alone will not be enough.

    By emphasizing consistent mask-wearing, incentivizing vaccination among employees, keeping better track of close contacts, or decreasing the size of cohorts that participate in unmasked activity together, you can decrease your organization’s reliance on testing as a mitigation measure. Use the Calculator to model various scenarios and predict how improvements in these areas can change your test recommendations.

  • When should I use Hotspot conditions?

     

    The answer to this question depends on whether your organization has already done baseline COVID-19 testing of your population. Baseline testing means that you’ve tested 100% of your population before starting a regular COVID-19 testing program. Doing baseline testing will help you get the most accurate results from the When to Test Calculator.

    If your organization has already done baseline testing
    If less than 1% of your population was positive on baseline testing, then use TYPICAL conditions. Otherwise, use HOTSPOT conditions.

    If your organization has not done baseline testing
    If baseline testing has not been performed, check your area's published test positivity rates. If those rates are greater than 1%, use HOTSPOT conditions.

  • When the Calculator says my organization only needs symptomatic screening, it also says I don't need any staff and that my costs are "N/A." Doesn't symptomatic screening require staff and resources, too?

     

    The When to Test Calculator is designed to assess only your organization's need for asymptomatic screening: regular, ongoing testing of individuals who do not have symptoms of COVID-19. If your organization does not need asymptomatic screening, then no resources or staff for that type of screening are required.

  • "Symptomatic Screening Only" appeared in the Frequency column of my calculator results table. What does that mean?

     

     

     

    The When to Test Calculator tells you how often your organization needs to perform what is called “asymptomatic screening,” or testing people who do not have COVID-19 symptoms. The goal of asymptomatic screening is to find and isolate infected people who do not have symptoms, in order to prevent them from transmitting the virus to others.

    If your conditions are favorable and your organization has excellent mitigation measures in place, you may not need to do asymptomatic screening in order to effectively reduce the chance of an outbreak. In those cases, the Calculator will recommend “Symptomatic Screening Only,” meaning that you should continue to require COVID-19 testing for people who display symptoms, but you do not need to test those who are symptom-free.

    NOTE: The Calculator’s results are based upon the CDC’s most recent guidance regarding how transmissible the most common variants of the virus are. It’s a good idea to be prepared to institute asymptomatic screening should your organization’s situation change, and to frequently recheck the Calculator’s results for your organization.
     

  • Why does the Calculator recommend that we test more frequently if we're using antigen tests rather than PCR?

     

     

    In general, antigen tests are less sensitive than PCR tests. That means they are not as good as PCR tests are at finding people who aren't carrying as much virus, including children and asymptomatic adults. Testing more frequently helps to make up for the antigen tests’ lower sensitivity.

    In addition, people who were infected very recently often carry low levels of virus, so if you test them using an antigen test at that time, they may test negative. A few days later, when their virus levels have risen significantly, an antigen test will come back positive. Because PCR is more sensitive, it can catch an infection early in the disease course, eliminating the need for another test.

  • What are the main differences between the various test types listed in the calculator results?

     

    The amount of testing your organization needs to do depends in part on the type of test you use. The When to Test Calculator returns results for the test categories listed in the table below, as well as additional results for other pooled testing strategies.

    NOTE: The test categories shown in the When to Test Calculator are examples only; they do not represent any one individual test.

    For more information, see the following FAQ topics:

  • Why does the When to Test Calculator show me two sets of results?

     

    The Calculator is designed so that you can compare how your organization’s testing needs would change if you altered your mitigation strategies. It can also allow you to compare the testing needs of different cohorts of people within your organization.

  • Under the Implementation Details tab, what does "Recommended max days between tests/person" mean?

     

    Some people in your organization may not be on-site every day. Those people still need to be tested on essentially the same schedule as those who are onsite on a daily basis.

    For example, if the When to Test Calculator lists the recommended max days between tests/person as “7,” then all individuals in your organization should be tested once every seven calendar days, regardless of how frequently they come to your site.

    This calculation assumes that testing is uniformly spread across the testing period for onsite testing. If it is desired to complete all testing within a shorter period of time, then it may be necessary to hire more staff to conduct sampling and testing and to purchase additional instruments (if an instrument-based test is chosen).

  • I want to compare two different groups of people within my organization. How do I do that?

     

    If your organization includes groups of people whose mitigation strategies are distinctly different (for example, teachers vs. students, office workers vs. assembly-line workers), the When to Test Calculator can show you how those groups’ testing needs differ.

    Here’s an example of how to compare these two groups. The table on the bottom right of the screenshot below shows the Calculator’s results for the teachers in a school. There are 150 teachers (orange arrow), and 75% of them are fully vaccinated (green arrow). This scenario is named TEACHERS (blue arrows).

    Now we want to see the results for the students in this school, who have the same mitigation strategies in place except that only 25% of them are vaccinated. We enter the number of students in the school, and we change the Fully Vaccinated number to 25% (green arrows). That changes the results in the table on the bottom right and brings up the Name New Scenario box (blue arrow). We name this scenario STUDENTS, and save it by clicking Save As (orange arrow).When we click Save As, the new scenario has been named STUDENTS (blue arrows).

    Now we want to compare TEACHERS and STUDENTS side by side. To do that, we go to the drop-down menu above the table on the bottom left and choose TEACHERS (green arrow).Now we see the results for TEACHERS on the bottom left (green arrow) and STUDENTS on the bottom right (blue arrow).

     

  • My organization is a store, restaurant, doctor’s office, or other facility that serves clients in person. Can I use the When to Test Calculator to evaluate my testing needs?

     

    Yes: You can use the Calculator to evaluate the testing needs of your staff. When you input the size of your organization, do not include your clients or other visitors, only the members of your organization.

4. THE WHEN TO TEST CALCULATOR: ADVANCED SETTINGS

  • What is prevalence?

     

    For the purposes of the When to Test calculators, prevalence is the percentage of individuals in a population who are infected with COVID-19 at any given moment. If there are 100 people in a building and one person has COVID-19, then the prevalence of COVID-19 in that building is 1%.

     

    The Organizational Calculator’s prevalence default settings are 1% for typical conditions and 3% for hotspot conditions. To adjust these settings, click on the Show Advanced Settings button on the results page, open the Main Calculator Settings tab, and scroll down to the Prevalence settings (green arrows below).


    The Individual Calculator uses prevalence information that is estimated at the county level.