COVID: which test, when and why?
This week’s expert:
A/Prof Sanjaya Senanayake AO, Infectious Diseases Physician; Medical School, ANU
Drawn from an interview with A/Prof Senanayake by Dr Harry Nespolon on the Healthed podcast Going Viral.
• In an acute infection of COVID-19, the PCR test is most likely to be positive three days after the first symptoms develop.
• The PCR may be negative if done too early in the course of the disease, ie prior to symptoms developing.
• In some patients, it may be worthwhile repeating the PCR test if the initial test is negative but infection is thought to be possible. These would be patients who, because of history and/or developing symptoms, are suspected of having COVID-19. There are no data to guide the most appropriate interval between PCR tests, but it is generally thought that an interval of at least 24 hours would be reasonable.
• Increasingly, patients are being asked by their employer to produce a medical certificate of clearance that they don’t have COVID-19, including evidence of a negative PCR test before being allowed back to work. Such a certification has limitations, given the possibility of a false negative as mentioned earlier.
• The probability that a negative PCR test is a true negative largely depends on the pre-test probability that the patient has been exposed to COVID-19. In the current climate in Australia, with so few new cases currently being reported, an asymptomatic patient who has a negative PCR test has a negligible risk of having the virus.
• If the patient does have symptoms and/or there is a history of potential exposure, it would be best to have the patient self-isolate and then repeat the PCR test.
• While PCR test may not be perfect in terms of specificity and sensitivity, it is the preferred option for diagnosing acute infection over antibody testing, which to date is still too unreliable for use as a screening test for acute infection.
• Diagnosing an acute infection via an antibody test relies on the detection of IgM antibodies, however the timing of the appearance of IgM will vary between infected individuals.