1. COVID-19 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 or in a health-care provider's office, at a testing site, or in your home. 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.
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.
Prevalence is the percentage of people in a population who have a disease. So if there are 100 people in your area and 10 of them have COVID-19, the prevalence of COVID-19 in your area is 10%.
Technically, the only way to determine the true prevalence of COVID-19 in a population is to test every single person in the population to see who has it and who doesn't. Since that's not feasible, the Calculators use a sophisticated machine-learning model that looks at all of the available COVID-19 data for a particular state and county and develops a "predicted prevalence" for that location.
For US territories and areas outside the US, the Calculators use a prevalence of 1% for typical conditions, and 3% for hotspot conditions.
R0 (R-naught) is a measure of the average number of people who will contract a contagious disease from one infected person. It’s a measure of viral transmissibility, or how fast the disease is spreading.
When R0 is exactly 1.0, it means that, on average, one infected person will infect one other person. When R0 is less than 1.0, it means that the spread of the disease is decreasing within the community. When R0is above 1.0, the spread of disease is increasing. The more transmissible a disease or disease variant is, the higher its R0.
When you're using a test to find out if someone has a disease, there are four possible results you can get:
- True positive: The test result is positive and the person has the disease
- True negative: The test result is negative and the person doesn't have the disease
- False positive: The test result is positive but the person doesn't have the disease
- False negative: The test result is negative but the person does have the disease
In an ideal world, all tests would return only true positives and true negatives. Unfortunately, no test is perfect.
Both When to Test Calculators are based on CDC guidelines. Current guidelines define a “close contact” as someone who was within six feet of an infected person for a total of at least 15 minutes over the course of 24 hours. One exception to that rule is applicable only to schools: Students who were between three and six feet of an infected person are not considered close contacts as long as they were in school and “correctly and consistently” wearing masks.
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 for Organizations 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.
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.
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.
More detailed data about how sensitivity and specificity affect the rate of false positives and false negatives in pooled testing is available on the Group Testing website, developed by Paolo Bertolotti of IDSS at MIT.
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
Open and Safe Schools — Includes toolkit on how to start a pooled testing program at your school.
Group Testing — Detailed data on how pool size and test sensitivity and specificity affect the number of false positives and false negatives in a pooled testing system. Developed by Paolo Bertolotti of IDSS at MIT.
Variants are mutated forms of SARS-CoV-2, the virus that causes the disease COVID-19. The variants that cause concern are ones whose mutations make them more likely to spread from person to person, cause more severe disease, and/or are able to more easily infect people who have been vaccinated.
All COVID-19 tests that have Emergency Use Authorization (EUA) from the FDA must be checked on an ongoing basis to make sure that they are able to detect different variants of the virus. If a test has any difficulty detecting a variant, the FDA must be notified immediately.
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 for Organizations includes a default measurement of how transmissible the virus is. That measurement is called R0 (“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.
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 for Organizations identifies four testing strategies for follow-up testing a positive pool. Details on each strategy appear beneath the table below.
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
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.
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.
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 in schools, please download the When To Test K-12 Playbook.
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.
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.
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.
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.
If your organization is running tests in a central lab that you own (for example, a university lab), you should model your situation using off-site PCR tests.
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 Calculators assumes that tests that do not have FDA authorization for screening are used off-label under practitioner guidance.
In the US, the use of medical tests is regulated by the Food and Drug Administration (FDA). These tests can only be used in this country once they’ve undergone a long approval process. In early 2020, the US was in desperate need of tests for COVID-19; we needed them as soon as possible. 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 for Organizations assumes that your workplace or school is using a test that has received EUA.
Isolation and quarantine mean essentially the same thing and serve the same purpose: They help protect the public by preventing exposure to people who have or may have a contagious disease.
Isolation separates people who are infected from those who aren’t.
Quarantine separates people who were exposed to a contagious disease from others and restricts their movements while they wait to see if they are infected.
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.
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.
2. COVID-19 MITIGATION
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.
If you plan to test only the unvaccinated members of your organization, then the number you enter for the "Fully Vaccinated" input should be zero.
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.
Yes. The Calculator uses an estimate of how well the vaccines available in the US protect against the Delta variant, which is currently dominant in the country. This estimate is based on the performance of all available vaccines, not on any one vaccine brand or type.
If you are using the Calculator for Organizations and want to adjust the Calculator's default so that it matches a specific vaccine, go to the Advanced Settings, click on "Main Calculator Settings," and look for "Vaccine Effectiveness."
CDC — COVID-19 Vaccine Effectiveness Research
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 for Organizations 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.
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).
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 for Orgnanizations 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).
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 When to Test Calculator for Organizations 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.
Oregon Department of Education – Cohorting One example of a school-based cohorting system.
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 for Organizations 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).
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.
3. USING THE WTT CALCULATOR FOR ORGANIZATIONS
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 for Organizations 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 left of the screenshot below shows the Calculator’s results for the employees in one offfice of a company. There are 250 employees in the office; 188 of them (75%) are participating in the testing program (orange arrow), and 50% of them are fully vaccinated (green arrow).