Question: Curious about Trump’s COVID tests……If he tested positive when he was infected, how is it possible that the most recent test is negative, so soon after his hospitalization?

Answer: Short answer: It’s because they are using an antigen test.

Slightly longer answer: In order to identify when a positive person is no longer contagious, CDC recommends using symptoms-based strategy NOT a test-based strategy (Figure 1). The president’s doctors, however, seem to have used a test-based strategy to end his isolation. And the test they used is an antigen test, which is unlikely to present a positive result if 1+ week has passed since symptom onset (Figure 2).

Longest answer: Back on October 12th, the president’s doctor wrote a memo stating, “In response to your inquiry regarding the President’s most recent COVID-19 tests, I can share with you that he has tested NEGATIVE, on consecutive days, using the Abbott BinaxNOW antigen card.” More recently (October 22), White House chief of staff Mark Meadows told reporters that the president was tested aboard Air Force One en route to the debate and that the test came back negative. Given the circumstances of that most recent test, coupled with the 10/12 memo and what we know of the current White House testing strategy, I think we can confidently assume that the test aboard Air Force One was also the Abbott BinaxNOW antigen test. We talked about antigen tests, and the BinaxNOW tests specifically, in our Q&A of 9/18 and Q&A of 9/16 #Antigen Test. Basically, the antigen test will pick up the virus and show a positive result if you are in the late pre-symptomatic or early symptomatic phases of COVID. However, about 1 week post-symptom onset and thereafter, the antigen test will be increasingly unable to detect the presence of viral proteins and will return a negative result (Figure 2). On the flip side, had PCR tests been used, there’s a strong possibility that they would still return as positive because they can capture SARS-CoV-2 RNA for a long period of time, much longer than a person is contagious. For these reasons, CDC “does not recommend using antigen tests to make decisions about discontinuing isolation.” Furthermore, CDC’s interim guidance states, “A [PCR] test-based strategy is no longer recommended (except as noted below) because, in the majority of cases, it results in prolonged isolation of patients who continue to shed detectable SARS-CoV-2 RNA but are no longer infectious.” So really when it comes to understanding whether the president was/is contagious while interacting with others, we’d need to know timing of symptom onset, timing of last fever, and symptom improvement.

Figure 1. Symptoms-based strategy for identifying when a patient is no longer at risk of transmitting the virus to others (from CDC)

Figure 2. Probability of Positive Result by Test Type (from Nature)