Question: Reflecting back on the Q&A of 9/16, I thought I remembered the governor of Ohio getting a false positive when he was set to greet the president at the airport. He then got tested two more times and tested negative, and he gave people a heads up about the rapid tests not being accurate. That being said, does it make sense to go once a week to get a rapid test? Or is it just a waste of time and resources?

Answer: Your memory is spot on. No test is perfect, and the likelihood of an antigen test giving a false positive depends on the prevalence of infection among the testing population. Personally, I support frequent, widespread use of the antigen tests AND I want to remind us all that we cannot rely on testing only. A negative test result does not give license to forgo other key public health measures, like social distancing, mask wearing, hand washing and the like. Read on for more detail and data!

The Abbott test we discussed in our Q&A of 9/16 #Antigen Tests, reportedly has a “sensitivity of 97.1% (positive percent agreement) and specificity of 98.5% (negative percent agreement) in patients suspected of COVID-19 by their healthcare provider within the first seven days of symptom onset.” As Table 1 shows, as overall prevalence increases, the possibility of false positives declines and the possibility of false negative increases. And as Figure 1 shows, antigen tests are only strong at identifying true positives when viral load is at its peak, relatively early on in infection. For these reasons, it’s especially important to work with your health provider to determine if you need another test (i.e. a PCR test) after having an antigen test, especially if you are asymptomatic and receive a positive antigen result or symptomatic and receive a negative antigen result.

When it comes to how frequently to test, a couple of days ago, Nature published a good overview, “Fast coronavirus tests: what they can and can’t do.” As described in the article, a couple of new pieces of research have shown the benefit of widespread rapid testing conducted frequently (even as often as 2x/week) to curb the spread of the epidemic. There’s no guidance (to my knowledge) on widespread population-based testing. Meanwhile, CDC suggests, “Rapid antigen tests can be used for screening testing in high-risk congregate settings in which repeat testing could quickly identify persons with a SARS-CoV-2 infection to inform infection prevention and control measures, thus preventing transmission.” CDC also offers guidelines for antigen testing in nursing homes. Others are pushing use of antigen testing to curb transmission in colleges and HHS recently suggested using antigen testing for use in schools (for more on #Universal Testing and schools, see Q&A of 9/1). While many folks have reservations about antigen tests in various scenarios (for more, see Nature article referenced above), public health experts agree that more widespread testing is absolutely necessary to curb the spread of the virus.

Table 1. Abbott Antigen Sensitivity/Specificity in Different Scenarios

Figure 1. Probability of Positive Result by Test Type (from Nature)