Antigen tests for COVID-19 can yield false positive results, especially when used improperly, said the US Food and Drug Administration (FDA) in a notification sent to clinical laboratory staff and health care providers.
“Remember that positive predictive value (PPV) varies with disease prevalence when interpreting results from diagnostic tests,” said FDA in the 3 November letter. Lower disease prevalence in a given area or population increases the number of false positive results a test will return, explained the agency.
“Like molecular tests, antigen tests are typically highly specific for the SARS-CoV-2 virus. However, all diagnostic tests may be subject to false positive results, especially in low prevalence settings,” added FDA.
By way of example, the letter looks at a hypothetical test with 98% specificity. When prevalence of a disease or condition is 10%, that test has 80% positive predictive value (PPV) and will return 20 false positive results for every 100 positive test results. However, when the same test is used in the scenario of 1% prevalence, PPV drops to just 30%; in this case, 70 out of every 100 positive tests are false positives…