Longitudinal SARS-CoV-2 testing is punctuated by intermittent positivity and variable rates of cycle-threshold decline

ABSTRACT

The COVID-19 pandemic is complicated by cases of vaccine-breakthrough, re-infection, and widespread transmission of variants of concern (VOC). Consequently, the need to interpret longitudinal positive SARS-CoV-2 (SCV-2) tests is crucial in guiding clinical decisions regarding infection control precautions and treatment. Although quantitative tests are not routinely used diagnostically, standard diagnostic RT-PCR tests yield Ct values that are inversely correlated with RNA quantity. In this study, we performed a retrospective review of 72,217 SCV-2 PCR positive tests and identified 264 patients with longitudinal positivity prior to vaccination and VOC circulation. Patients with longitudinal positivity fell into two categories short-term (207, 78%) or prolonged (57, 22%) positivity, defined as <= 28 (range 1-28, median 16) days and >28 (range 29-152, median 41) days, respectively. In general, Ct values declined over time in both groups; however, 11 short-term positive patients had greater amounts of RNA detected at their terminal test compared to the first positive, and 5 patients had RNA detected at Ct < 35 at least 40 days after initial infection. Oscillating positive and negative results occurred in both groups, although oscillation was seen three times more frequently in prolonged-positive patients. Patients with prolonged positivity had diverse clinical characteristics but were often critically ill and were discharged to high-level care or deceased (22%). Overall, this study demonstrates that caution must be emphasized when interpreting Ct values as a proxy for infectivity, predictor of severity, or a guide for patient care decisions in the absence of additional clinical context.