FDA diagnostics guidance updated to include antigen testing parameters

With the first antigen test for COVID-19 authorized over the weekend, the FDA released an updated guidance document containing information on how companies can submit an antigen test for emergency authorization.


By Laura DiAngelo, MPH

How Things Work Now

Since the declaration of a Public Health Emergency (PHE) for COVID-19 in February, the FDA has issued Emergency Use Authorizations to more than 100 products intended to address the viral condition. To support diagnostic testing for the novel coronavirus, the FDA issued EUA “templates” for developers to streamline the regulatory review and authorization process. So far, there have been EUA templates for molecular diagnostics and serological tests.


What’s New

On May 11, the FDA updated its diagnostic guidance with information on how to submit a third type of diagnostic for an EUA: Antigen tests. While serological tests are intended to detect antibodies to the virus (i.e., the individual’s immune response) and molecular diagnostic can detect the presence of viral RNA, antigen tests detect viral proteins. These tests are less complex than molecular diagnostics, but more reliable than serological tests. The FDA authorized the first SARS-CoV-2 antigen test over the weekend, the Sofia 2 SARS Antigen FIA from Quidel Corporation.


The agency also posted an EUA template for developers to use when designing their validation programs. The template includes additional explanations on how validation studies and the types of samples that the FDA will consider acceptable in testing.


For antigen studies, the following validation studies should be performed:

  • Limit of Detection (LoD)/Analytical Sensitivity: For antigen tests, the FDA will consider LoD as the lowest concentration at which 19/20 replicates are positive. The agency doesn’t recommend the use of recombinant antigens for the LoD determination, but instead urges developers to use spike inactivated virus samples in a real clinical matrix.

  • Cross-Reactivity/Analytical Specificity: The FDA recommends several high-priority organisms for cross-testing to ensure that the antigen test can appropriately distinguish against other pathogens or organisms in the same genetic family or that are likely to be circulating in the area. For wet testing, the FDA recommends cross-reactivity testing against three other types of coronavirus, adenovirus, human metapneumovirus, parainfluenza virus 1-4, influenza A&B, enterovirus, respiratory syncytial virus, rhinovirus, and several other similar organisms. For an in-silico protein blast analysis, the FDA recommends study of the test’s response to SARS-coronavirus, MERS-coronavirus, human coronavirus HKU1, mycobacterium tuberculosis, and pneumocystis jirovecii. If the test is being submitted for use in multiple matrices, the FDA recommends the developer choose the most complex matrix in which to perform this testing.

  • Microbial Interference Studies: If a developer’s cross-reactivity studies have not yielded any evidence that the assay will not react to any of the high priority organisms or pathogens as the novel coronavirus, the FDA recommends that the developer conduct microbial interference studies to ensure that a high volume of these organisms will not cause a false negative for samples with both, or multiple, pathogens present. The FDA recommends that these studies be conducted with low concentrations of the novel coronavirus and high concentrations of the interferent to represent a worse-case scenario.

  • Endogenous Interference Substance Studies: Developers should provide studies on whether a patient’s natural, endogenous substances (e.g., antibodies) could interfere with the test. However, the types of studies will depend on the matrices (e.g. sputum) that the developer intends for use with their device.

  • Clinical Evaluation: Developers should test their product on a minimum of 30 positive and 30 negative specimens in a randomized, blinded study. The use of contrived clinical specimens will not be considered acceptable. If the developer wants their tests indicated for saliva (or other oral fluid), they should test at least 30 positive specimens, pared with PCR results from a nasopharyngeal swab. The agency also recommends testing for the high-dose hook effect (i.e., the test may product a false negative if the target is present in very high concentrations), matrix equivalency, and specimen stability. If the device would be intended for use at the point of care, sponsors should also demonstrate that non-laboratory personnel can effectively use the device.



With a third test-type available, testing capacity could increase. However, antigen test developers are likely to face the same challenges as molecular diagnostic kit developers in ramping up development to meet the need. One key issue for testing capacity has been the availability of accessories, such as swabs and re-agents.

Demand for both has skyrocketed as COVID-19 spread. A new White House policy to distribute almost 3 million swabs and about 10 million tubes of reagents could help mitigate the issue. New FDA authority requiring medical device manufacturers to report certain shortages could help the FDA gain insight into (and take action on) limited supplies of these products, though the FDA is presumably already aware of the shortages and trying to take action.


What’s Next

Manufacturers of antigen test manufacturers will be able to market and distribute their test for up to 15 days before submitting an EUA as long as they validate the test and notify the FDA. That timing is somewhat longer than the 10 days allowed for serological test products to do the same.


Tests for which the FDA does not issue an EUA after reviewing the submission, or those that are not submitted within the timeframe, will be removed from the FDA’s listing of notified tests. The FDA said it would take “additional action” as necessary.


With the White House announcing policies to encourage states to test “at least 2% of their populations” in May, developers are likely to see continued exponential demand growth this month.


To contact the author of this analysis, please email Laura DiAngelo.
To contact the editor of this analysis, please email Alec Gaffney.


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