What are coronavirus disease 2019 (COVID-19) antigen detection tests?

Updated: Apr 02, 2021
  • Author: James J Dunn, PhD, D(ABMM), MT(ASCP); more...
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As of October 26, 2020, the FDA had issued EUA certification for six commercially available SARS-CoV-2 antigen detection tests. [9] These are typically lateral-flow immunoassays intended for the qualitative detection of nucleocapsid protein antigen directly from NP and/or nasal swabs; the presence of such antigen implies current SARS-CoV-2 infection. Several of the antigen test systems require an instrument reader to determine the results, whereas others can be read visually by the operator.

In general, the workflow of the antigen detection test requires placement of the swab into a reagent solution that is mixed and then applied to a test cartridge. The sample is then allowed to migrate along the test strip, where, if the antigen is present, specific antibodies to SARS-CoV-2 antigen will react with the sample and produce a colorimetric or fluorescent signal within 10-15 minutes. All test systems require use of the swab provided in the kit, and in most cases it is applied directly without dilution in VTM. The tests include built-in internal controls that must be visualized by the operator or read by the instrument to ensure the validity of results. In all cases, the manufacturer’s IFU should be strictly followed.

Antigen detection tests with EUA from the FDA are authorized for diagnostic testing in symptomatic individuals within the first 5-12 days of symptom onset. It is best to perform this type of testing in the early stages of infection, when the viral load is generally highest, since antigen levels in specimens collected beyond 5-7 days post symptom onset may drop below the assay's detection limit. There is scant information on the utility of antigen testing for SARS-CoV-2 in asymptomatic persons. At this time, the CDC recommends that antigen tests not be used to make decisions about discontinuing isolation. [52]

From a regulatory standpoint, all of the EUA antigen detection tests can be employed by laboratories certified under CLIA to perform moderate-, high-, or waived-complexity tests and at the point of care in patient care settings operating under a CLIA Certificate of Waiver, Certificate of Compliance, or Certificate of Accreditation. In other words, the testing does not necessarily need to be performed in a laboratory by a trained staff of technologists but can be used in other settings (eg, physicians' offices) that have regulatory approval to provide this simple type of assessment. Regardless of where they are performed, CLIA-certified laboratories or performing sites within the United States are required to report all positive results to the appropriate public health authorities. [52]

On December 15, 2020, the FDA granted an EUA for the first over-the-counter COVID-19 test that can be performed fully at home. A rapid, lateral flow antigen test, the Ellume COVID-19 Home Test produces results in as little as 20 minutes. In symptomatic individuals, the test was found to correctly identify 96% of positive samples and 100% of negative samples, while in asymptomatic persons, the rates were 91% and 96%, respectively. Authorization has been given for individuals aged 2 years and older, whether or not they are symptomatic. [53]

Another at-home antigen test, the BinaxNOW COVID-19 Ag Card Home Test, this one available by prescription, was given an FDA EUA on December 16, 2020. It is authorized for use in individuals aged 4 years or older in whom COVID-19 is suspected, with the test to be performed within 7 days of symptom onset. [54]

On April 1, 2021, the FDA authorized the antigen tests Quidel QuickVue At-Home OTC COVID-19 test, Abbott BinaxNOW COVID-19 Antigen Self Test, and Abbott BinaxNOW COVID-19 Ag Card 2 Home Test for nonprescription, at-home use in the serial screening of asymptomatic persons for SARS-CoV-2 infection. In addition, the Ag Card 2 test and another antigen test, the BD Veritor System for Rapid Detection of SARS-CoV-2, were authorized as point-of-care tests for serial screening of people without symptoms, although the BD Veritor System requires a prescription. [55]

Although antigen detection tests are simple, easy to perform, and fairly inexpensive, one of the major concerns associated with their use is the lack of analytic and clinical sensitivity compared with RNA detection tests. A meta-analysis of four commercially available antigen tests available outside of the United States showed that the average sensitivity was 56.2%. [56] In a study using HCP-collected nasal swabs in two EUA antigen tests, overall agreement between them was 98.1%, while, compared with RT-PCR, one of the tests displayed a positive percent agreement (PPA) of 82.4% and a negative percent agreement (NPA) of 99.5%, in adult patients with onset of symptoms less than 7 days prior to collection. [57]  

Due to these limitations, it may be necessary to confirm a negative antigen test with NAAT when the pretest probability is relatively high, especially if the patient is symptomatic or has a known exposure to a confirmed COVID-19 case. A negative antigen test does not necessarily need to be confirmed by NAAT if the pretest probability is low, as in cases in which the individual is asymptomatic or has no known exposures. [52] Generally speaking, interpretation of the results from antigen detection tests will depend on the local prevalence of COVID-19, the assay performance characteristics (ie, sensitivity/specificity), and the patient’s clinical signs, symptoms, and history.

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