Covid test: Time of day you are tested affects accuracy
The time of day you have your Covid test affects the accuracy, new research shows, but it comes with another twist.
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The best time to get an accurate Covid test result is 2pm – but it’s also the time of day you are most likely to spread the virus to others, a new study has found.
Researchers from Vanderbilt University in Nashville have found you are almost twice as likely to get an accurate positive result if you get tested in the early afternoon rather than at night.
Dr Carl Johnson studied 86,342 Covid PCR tests performed on symptomatic and asymptomatic patients in 130 clinics in the south eastern area of the US from March to August 2020.
The study published in the Journal of Biological Rhythms found the proportion of positive tests varied depending on the time of day they were done with tests taken at mid-afternoon 1.7 times more likely to be positive than those performed after 8pm.
An individual’s viral load becomes lower after 8pm, he explained.
This means that the result from a test taken at that time of day is more likely to be a false negative, the study found.
“False negatives can be harmful to the community and for the patient, who might not seek additional care due to their result,” Dr Johnson told Vanderbilt University News.
The study findings suggests people with the virus are also likely to be most infectious and spread the disease more easily to others during the middle of the day.
This is a concern because it’s when they are most likely to interact with other people or access healthcare, Dr Johnson said.
The reason for the variation is that the body clock – our circadian rhythm – influences the behaviour of the virus and the immune system over a 24-hour cycle.
Other studies of viruses and bacterial infections back the hypothesis that viral shedding is highest in the middle of the day, he said.
Viral shedding is the process where the virus is released into the blood and the mucus and can be spread to others when you breathe, talk or sneeze.
Dr Johnson speculates that perhaps the virus sheds most at a time of day when the immune system is least able to deal with it.
The findings could be an important guide for doctors trying to manage Covid patients
“Taking a Covid-19 test at the optimal time of day improves test sensitivity and will help us to be accurate in diagnosing people who may be infected but asymptomatic,” he said.
This study focused on PCR tests taken by professionals but its findings imply that people using less accurate at home rapid antigen tests now available in Australia might want to consider performing the test in the middle of the day rather than at night.
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Originally published as Covid test: Time of day you are tested affects accuracy
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