The most commonly used test for COVID-19 may not always yield accurate results, and timing of the testing seems to matter significantly in the accuracy of the results, according to a study.
Researchers from Johns Hopkins University in the US noted that one of the most commonly used diagnostic tools, particularly during this pandemic, is the reverse transcriptase polymerase chain reaction test (RT-PCR).
The test uses a person's respiratory sample to detect viral particles and determine if the person may have been exposed to a virus, they said.
Laboratory professionals across the globe have used RT-PCR to find out if a person has been infected with SARS-CoV-2, the virus that causes COVID-19.
While these tests are important, the researchers found that the chance of a false negative result -- when a virus is not detected in a person who actually is, or recently has been, infected -- is greater than 1 in 5 and, at times, far higher.
They caution that the predictive value of these tests may not always yield accurate results, and timing of the test seems to matter greatly in the accuracy.
The study, published in the journal Annals of Internal Medicine, found that the probability of a false negative result decreases from 100 per cent on Day 1 of being infected to 67 per cent on Day 4.
The false negative rate decreased to 20 per cent on Day 8, three days after a person begins experiencing symptoms, the researchers said.
They also found that on the day a person started experiencing actual symptoms of illness, the average false negative rate was 38 per cent.
In addition, the false negative rate began to increase again from 21 per cent on Day 9 to 66 per cent on Day 21.
The study, which analysed seven previously published studies on RT-PCR performance, adds to evidence that caution should be used in the interpretation of negative test results, the researchers said.
This holds particularly true for individuals likely to have been exposed or who have symptoms consistent with COVID-19, they said.