The agency received reports of false positives on these devices from nursing homes and other facilities and is monitoring the situation and evaluating the data.
“The FDA reminds clinical laboratory staff and health care providers about the risk of false positive results with all laboratory tests,” the statement said. “Laboratories should expect some false positive results to occur even when very accurate tests are used for screening large populations with a low prevalence of infection.”
Rapid tests quickly detect protein fragments in or on the virus by analyzing samples collected from the nasal cavity.
Antigen testing is an important tool because one of the main advantages of an antigen test is the speed of the test, which can provide results in minutes in a huge number of people.
However, rapid antigen tests are not as accurate as molecular tests and therefore it is recommended that a molecular test be performed before starting the patient’s treatment.
The FDA statement includes a list of the handling errors that can result in false positives, including cross-contamination between one sample and another, lack of sanitation, not changing gloves between tests, not respecting the times indicated in the instructions, or performing too many tests together.
The agency said it is working with the manufacturers of these tests to make the instructions as clear as possible.
The FDA concluded by explaining how the number of false positives in a sample are measured, “The positive predictive value (PPV) varies with disease prevalence when interpreting results from diagnostic tests. PPV is the percentage of positive test results that are true positive. As disease prevalence decreases, the percentage of test results that are false positives increase.
“For example, a test with a specificity of 98% would have a PPV of just over 80% in a population with a prevalence of 10%, meaning that 20 out of every 100 positive results would be false positives. …
“This means that in a population with 1% prevalence only 30% of individuals who test positive actually have the disease.”