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Performance of an Antigen-Based Test for Asymptomatic and Symptomatic SARS-CoV-2 Testing at Two University Campuses — Wisconsin, September–October 2020

Our take —

SARS-CoV-2 antigen-based testing using the Sofia SARS Antigen Fluorescent Immunoassay demonstrated good diagnostic accuracy in this study of 1,098 paired samples from two universities in Wisconsin, and thus may aid in serial testing in congregate settings such as universities. Confirmatory RT-PCR testing is recommended in certain instances in order to rule out false positive or false negative test results. The study population was predominantly white and under the age of 24 years, thus potentially limiting the generalizability of these findings.

Study design

Cross Sectional

Study population and setting

Performance data on an FDA-authorized antigen-based SARS-CoV-2 test (Sofia SARS Antigen Fluorescent Immunoassay, Quidel Corp.) relative to CDC RT-PCR test was obtained using 1,098 paired nasal swab samples from individuals at two universities in Wisconsin, from September to October 2020. Subjects included both sexes and were predominately white students under the age of 24 years, either with (20.7%), or without (79.3%) self-reported COVID-19-compatible symptoms at the time of sample collection.

Summary of Main Findings

Compared to the CDC RT-PCR test, in subjects reporting one or more symptoms at the time of sample collection, the antigen-based test had a sensitivity of 80.0%; a specificity of 98.9%; positive predictive value (PPV) of 94.1% and negative predictive value (NPV) of 95.9% in this population. For asymptomatic subjects, sensitivity was 41.2%; specificity was 98.4%; PPV was 33.3% and NPV was 98.8%. Negative predictive value is the probability of not having disease given a negative test, while positive predictive value is the probability of having disease given a positive test.

Study Strengths

The primary study strength was that these findings were based on real-world data collected from university-based testing programs. This study also directly compared assay performance of an FDA-approved antigen-based SARS-CoV-2 test to CDC RT-PCR test using paired, contemporaneously collected nasal swab samples. For a subset with positive RT-PCR or antigen test results, viral culture was performed to further validate findings.

Limitations

Test subjects were predominately white (84%), college-age students, from a single university (96%), thus potentially limiting the generalizability of these study results to other populations. The study population was already undergoing serial testing, thus the NPVs and PPVs obtained here would not necessarily apply to populations undergoing less intensive screening. Additionally, the study only evaluated the performance of the Sofia SARS Antigen FIA, thus these results cannot be generalized to other antigen-based tests.

Value added

This study supplied performance data for an antigen-based SARS-CoV-2 test. Compared to RT-PCR, antigen-based tests are faster and cheaper, and thus have potential advantages for use in serial testing of congregate populations. These results indicate that the Sofia SARS Antigen FIA test may have utility in serial testing of young adults in the university setting, but that confirmatory nucleic acid-based testing should be considered for negative test results in symptomatic individuals and for positive test results in asymptomatic individuals, in order to help rule out potential false negatives and false positives, respectively.

This review was posted on: 12 March 2021