The Emergency Use Authorization (EUA) authority allows FDA to help strengthen the nation’s public health protections against chemical, biological, radiological, and nuclear (CBRN) threats including infectious diseases, by facilitating the availability and use of medical countermeasures (MCMs) needed during public health emergencies.
Under section 564 of the Federal Food, Drug, and Cosmetic Act (FD&C Act), when the Secretary of HHS declares that an emergency use authorization is appropriate, FDA may authorize unapproved medical products or unapproved uses of approved medical products to be used in an emergency to diagnose, treat, or prevent serious or life-threatening diseases or conditions caused by CBRN threat agents when certain criteria are met, including there are no adequate, approved, and available alternatives. The HHS declaration to support such use must be based on one of four types of determinations of threats or potential threats by the Secretary of HHS, Homeland Security, or Defense.
Please note: a determination under section 319 of the Public Health Service Act that a public health emergency exists, such as the one issued on January 31, 2020, does not enable FDA to issue EUAs. On February 4, 2020, the HHS Secretary determined that there is a public health emergency that has a significant potential to affect national security or the health and security of United States citizens living abroad, and that involves the virus that causes COVID-19. Subsequent HHS declarations supporting use of EUAs and based on this determination are described in the blue boxes below.
Serology tests detect the presence of antibodies in the blood from the body’s adaptive immune response to an infection, like COVID-19. They do not detect the virus itself. In the early days of an infection when the body’s adaptive immune response is still building, antibodies may not be detected. This limits the test’s effectiveness for diagnosing current COVID-19 and is one reason serology tests should not be used to diagnose or exclude acute COVID-19 infection. Serology tests play a role in the fight against COVID-19 by helping health care professionals identify individuals who may have developed an adaptive immune response to SARS-CoV-2.
However, to use serology tests properly, it is important to understand their performance characteristics and limitations. Moreover, studies are underway to address questions that will better inform the appropriate use of these tests, such as which antibodies may indicate a level of protection that would prevent or reduce the severity of infection or re-infection as well as the duration for which this protection may last.
The performance of these tests is described by their “sensitivity,” or their ability to identify those with antibodies to SARS-CoV-2 (true positive rate), and their “specificity,” or their ability to identify those without antibodies to SARS-CoV-2 (true negative rate).
A test’s sensitivity can be estimated by determining whether or not it is able to detect antibodies in blood samples from patients who have been confirmed to have COVID-19 with a nucleic acid amplification test, or NAAT. In some validation studies of these tests, like the one the FDA is conducting in partnership with the National Institutes of Health (NIH), the Centers for Disease Control and Prevention (CDC), and the Biomedical Advanced Research and Development Authority (BARDA), the samples used, in addition to coming from patients confirmed to have COVID-19 by a NAAT, may also be confirmed to have antibodies present using other serology tests.
A test’s specificity can be estimated by testing large numbers of samples collected and frozen before SARS-CoV-2 is known to have circulated to demonstrate that the test does not produce positive results in response to the presence of other causes of a respiratory infection, such as other coronaviruses.
These estimates of sensitivity and specificity are just that: estimates. They include 95% confidence intervals, which are the range of estimates we are about 95% sure a test’s sensitivity and specificity will fall within given how many samples were used in the performance validation. The more samples used to validate a test, the smaller the confidence interval becomes, meaning that we can be more confident in the estimates of sensitivity and specificity provided.
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Tests are also described by their Positive and Negative Predictive values (PPV and NPV). These measures are calculated using a test’s sensitivity, its specificity, and using an assumption about the percentage of individuals in the population who have antibodies to SARS-CoV-2 (which is called “prevalence” in these calculations).
Every test returns some false positive and false negative results. The PPV and NPV help those who are interpreting these tests understand, given how prevalent individuals with antibodies are in a population, how likely it is that a person who receives a positive result from a test truly does have antibodies to SARS-CoV-2 and how likely it is that a person who receives a negative result from a test truly does not have antibodies to SARS-CoV-2.
The PPV and NPV of a test depend heavily on the prevalence of what that test is intended to detect. Because all tests will return some false positive and some false negative results, including tests that detect antibodies to SARS-CoV-2, broad use of the tests, when not appropriately informed by other relevant information, such as clinical history or diagnostic test results, could identify too many false-positive individuals.
The prevalence of SARS-CoV-2 antibody positive individuals varies across the United States, and prevalence is changing as the virus infects more individuals and as more individuals are vaccinated. In low prevalence populations, the result of a single antibody test is not likely to be sufficiently accurate to make an informed decision regarding whether or not an individual has had a prior infection or truly has antibodies to the virus. A second test, typically one assessing for the presence of antibodies to a different viral protein, generally would be needed to increase the accuracy of the overall testing results.
Methodology
For this page, the FDA has summarized the expected performance of the tests it has authorized based on the information the FDA reviewed when deciding whether or not to grant these tests an Emergency Use Authorization and with prevalence assumptions ranging from 5% to 50% for PPV and NPV calculations. For tests that had multiple validation studies or where the tests showed variable performance in samples collected at different times after symptom onset, the FDA analysts selected the results they considered to be most representative of expected test performance.
This is an incomplete representation of the performance of these tests. Always refer to the complete instructions for use to put these estimates into the proper context and to understand how to use and interpret these tests. The FDA is also providing a calculator that will allow users to see the estimated performance of a single test or two independent tests based on their performance characteristics and the estimated prevalence of SARS-CoV-2 antibodies in the target population.
Test Performance
Abbott AdviseDx SARS-CoV-2 IgG II (Alinity)
Developer: Abbott Laboratories Inc.
Test: AdviseDx SARS-CoV-2 IgG II (Alinity)
Technology: Semi-quantitative High Throughput CMIA
Target: Spike
| Antibody | Performance Measure | Estimate of Performance | 95% Confidence Interval |
|---|---|---|---|
| IgG | Sensitivity (PPA) | 98.1% (51/52) | (89.9%; 99.7%) |
| IgG | Specificity (NPA) | 99.6% (2000/2008) | (99.2%; 99.8%) |