A Closer Look at COVID-19 Diagnostic Testing
This overview may be of interest to health care providers, test purchasers, and public health staff who want to know more about different types of tests.
The FDA has been working around the clock to help increase the availability of critical medical products, including diagnostic tests, to fight the coronavirus disease 2019 (COVID-19) pandemic.
A patient and consumer overview1 of COVID-19 testing has plain language information about both diagnostic and antibody testing for COVID-19. This companion resource takes a closer look at diagnostic testing for COVID-19 and may be of interest to health care providers, test purchasers, and other public health professionals.
COVID-19 Diagnostic Tests
In certain types of emergencies, the FDA can issue an emergency use authorization, or EUA, to provide more timely access to critical medical products (including medicines and tests) that may help during the emergency when there are no adequate, approved, and available alternative options. The FDA has authorized for emergency use many tests2 that can diagnose infection with the virus that causes COVID-19, severe acute respiratory syndrome coronavirus 2 or SARS-CoV- 2.
Emergency Use Authorization
The EUA process is different than FDA approval, clearance, or licensing because the EUA standard is more flexible than the full approval, clearance, or licensing standard. Under an EUA, the data must show that a product may be effective and that the known and potential benefits outweigh the known and potential risks. This enables the FDA to authorize the emergency use of medical products that meet the criteria within days or weeks rather than months to years.
The FDA has prioritized review of EUA requests for tests where authorization would increase testing accessibility or would significantly increase testing capacity. As a result, the FDA prioritizes review of EUA requests for POC tests; home collection tests; at-home tests (none of which have been authorized as of October 2020); tests that reduce reliance on test supplies; and high-throughput, widely distributed tests.
Test Performance
No test is 100% accurate, and test performance can vary based on the prevalence of disease in the population being tested. COVID-19 diagnostic tests may be less accurate in populations with a low prevalence of disease and in asymptomatic individuals, individuals who shed little virus, or individuals who are early or late in the course of illness.
Tests are assessed based on their sensitivity and specificity. A test’s sensitivity is the fraction of positive cases that the test correctly identifies as positive, and a test’s specificity is the fraction of negative cases that the test correctly identifies as negative.
- A highly sensitive test will generally have a low false negative rate but will run a risk of false positives if the test’s specificity is low.
- A highly specific test will generally have a low false positive rate but will run a risk of false negatives if the test’s sensitivity is low.
To help mitigate the impact of false results, all COVID-19 tests authorized to date are prescription-only, so that clinicians can interpret results for patients.
Molecular versus Antigen Tests
Currently authorized SARS-CoV-2 diagnostic tests operate using one of two different underlying technological principles. These two diagnostic test types are molecular tests and antigen tests. Each type of test detects a different part of the SARS-CoV-2 virus particle.
- Molecular tests detect the genetic material or nucleic acid present inside a virus particle. The FDA has authorized molecular tests for use in a clinical laboratory setting and authorized some for use in a POC setting. Most molecular tests are polymerase chain reaction (PCR) tests, also called nucleic acid amplification tests (NAAT). In PCR testing, a machine located in a laboratory or at a POC, depending on the test, runs a series of reactions. These reactions first convert the virus’s ribonucleic acid (RNA), if present, into deoxyribonucleic acid (DNA) and then amplify it (make millions of copies of the DNA); the test then detects this DNA. By running multiple amplification cycles, a PCR test can sense even low levels of viral genetic material in a patient’s sample, so these tests tend to be highly sensitive (especially laboratory PCR tests).
- A PCR test can be authorized to run batched or “pooled” patient samples if the developer demonstrates that the test meets the EUA standard for pooled testing. If a pooled sample tests positive, the samples that were combined then need to be tested individually to identify the positive case(s). When there is a low prevalence of cases (and a high number of negative results is therefore expected), pooling samples may result in fewer tests needing to be run and fewer testing supplies being required.
- Antigen tests detect one or more specific proteins from a virus particle. All currently-authorized antigen tests are POC tests and provide results in less than an hour. Antigen tests tend to be highly specific but are typically less sensitive than molecular tests. However, because antigen tests can generally be produced at a lower cost than molecular tests and have a simpler design, antigen tests could scale to test millions of individuals per day.
| Molecular Test | Antigen Test | |
|---|---|---|
| Detects | Viral genetic material, through multiple amplification cycles in PCR testing | Protein(s) from a virus particle |
| Sample type | Nasal swab, nasopharyngeal swab, mid-turbinate swab, respiratory aspirate/lavage, or saliva sample, depending on the test | Nasal swab or nasopharyngeal swab, depending on the test |
| Laboratory or point-of-care (POC) | Most tests are authorized for use in laboratories, with certain laboratory tests authorized for a patient’s sample to be collected at home and then mailed to the laboratory for analysis. A few tests are authorized for use at the point-of-care. | As of October 2020, all currently-authorized tests are for use at the point-of-care |
| Turnaround time | Several hours to days for laboratory tests; less than an hour for point-of-care tests | Less than an hour |
| Sensitivity and Specificity | Highly sensitive (especially laboratory PCR tests) and highly specific | Highly specific, but generally less sensitive than molecular tests |
Conditions for Use
When authorizing a product for emergency use, the FDA issues a Letter of Authorization.3 Each Letter of Authorization issued by FDA for a COVID-19 product is available on the agency’s website. These letters set forth the conditions of authorization, including the setting for conducting the test, the indications for using the test, and the responsibilities of the test’s distributors and users.
Laboratory versus Point-of-care Tests
The vast majority of SARS-CoV-2 diagnostic tests with EUAs are authorized for use in laboratories certified under the Clinical Laboratory Improvement Amendments of 1988 (CLIA), 41 U.S.C. §263a, to perform high or moderate complexity tests. Laboratory tests include some tests for which an individual’s nasal swab or saliva sample can be self-collected at home and then mailed to an authorized laboratory for analysis. However, as of October 2020, the FDA has not issued an EUA for any test for which the patient’s sample can be collected and tested for the virus at home. Laboratory tests are authorized either for the specific laboratory in which the test was developed or for any laboratory that meets certain requirements set forth in the EUA.
Several SARS-CoV-2 diagnostic tests are authorized to be conducted entirely at the point-of-care (POC) without a sample being sent to a laboratory for analysis. The term “point-of- care” refers to a patient care setting, such as any of the following that meets certain requirements:
- Doctors’ offices
- Nursing homes
- Urgent care centers
- Pharmacies
- School nurse offices
- Workplace health clinics
For POC tests, each test authorized to date has been authorized for use in patient care settings operating with a Certificate of Waiver, a Certificate of Compliance, or a Certificate of Accreditation under CLIA. The Centers for Medicare & Medicaid Services (CMS) issues CLIA Certificates4 and enforces compliance with CLIA regulatory requirements.
Indications
A diagnostic test is authorized for use for the specific indications authorized in the EUA. To assist in the collection and submission of data and information needed to support an EUA request for a test, the FDA has provided templates on the agency’s website. The authorized indications for use vary from test to test. For example, a test may be authorized for one of the following categories of indications for use:
- Authorized for use with certain specimen types collected from individuals suspected of having COVID-19 by their health care provider, provided that the individuals have experienced the onset of symptoms of COVID-19 within a pre-specified number of days prior to administration of the test.
- Authorized for use with certain specimen types collected from individuals suspected of having COVID-19 by their health care provider, even if the individuals lack symptoms of COVID-19.
- Authorized for use with certain specimen types collected from any individual, including individuals without symptoms or any other reasons to suspect COVID-19 infection. Tests with this authorization are referred to as testing authorized for use in “asymptomatic screening.”
Screening Asymptomatic Patients
For licensed health care practitioners who are prescribing or administering an authorized SARS-CoV- 2 diagnostic test for asymptomatic individuals not suspected of having COVID-19, we recommend they consider the information below, as well as the HHS guidance on PREP Act coverage.5
Although the current available literature suggests that symptomatic individuals with COVID-19 and asymptomatic individuals without known exposure may have similar levels of viral genetic material, there is limited data on the distribution of viral loads in individuals with and without symptoms across demographics, different settings, and specimen types. Therefore, when screening asymptomatic individuals, health care providers should consider using a highly sensitive test, especially if rapid turnaround times are available. If highly sensitive tests are not feasible, or if turnaround times are prolonged, health care providers may consider use of less sensitive POC tests, even if they are not specifically authorized for this indication (commonly referred to as “off-label”). For congregate care settings, like nursing homes or similar settings, repeated use of rapid POC testing may be superior for overall infection control compared to less frequent, highly sensitive tests with prolonged turnaround times.
When using tests for general asymptomatic screening, health care providers should be aware of the performance of the tests and may want to consider different testing approaches, such as a predefined serial testing plan or directed testing of high-risk individuals. “Negative” results should be considered as “presumptive negative,” and health care providers should consider them in the context of clinical observations, patient history, and epidemiological information. Thus, if there is a significant new outbreak in a congregate care facility or high clinical suspicion of an infection in an individual resident, a negative POC test should be confirmed with a highly sensitive molecular test (refer to CDC guidelines). It is not necessary to perform confirmatory high-sensitivity molecular tests on individuals with negative antigen test or other POC test results if they are obtained during routine screening or surveillance.
1 https://www.fda.gov/consumers/consumer-updates/coronavirus-disease-2019-testing-basics
2 https://www.fda.gov/medical-devices/coronavirus-disease-2019-covid-19-emergency-use-authorizations-medical-devices/vitro-diagnostics-euas
3 https://www.fda.gov/medical-devices/coronavirus-disease-2019-covid-19-emergency-use-authorizations-medical-devices/vitro-diagnostics-euas
4 https://www.cms.gov/files/document/laboratory-quick-start-guide-cms-clia-certification.pdf
5 https://www.hhs.gov/sites/default/files/prep-act-coverage-for-screening-in-congregate-settings.pdf