COVID-19 test accuracy may vary by time of day

The accuracy of gold-standard PCR tests of nasopharyngeal swab samples may vary by time of day, new data suggest.

Researchers analyzed 31,094 tests performed in symptomatic and asymptomatic individuals at 127 testing sites, including 2,438 tests that showed COVID-19.

In a paper posted on Saturday on medRxiv ahead of peer review, they report tests were most likely to be positive around 2 p.m. – and the proportion of positive tests in the early afternoon was two-fold higher than the lowest proportion seen at other times of the day.

The study “suggests people may be more contagious at certain times of the day and it raises questions about whether tests for SARS-CoV-2 may be less accurate when they are collected between late evening and early morning,” said coauthor Dr. Candace McNaughton of Vanderbilt University.

“If our findings are confirmed, clinicians and public health teams could focus their efforts on lowering the risk of viral spread during times of peak viral shedding,” she said.

That could entail emphasizing mid-day to early-afternoon masking at home while isolating, or encouraging early morning shopping for vulnerable populations.

“There may be greater benefit in repeat testing if a negative test was collected when viral shedding is generally less,” McNaughton said.