COVID-19 Tests – Swab, Saliva, Serum, or Stool?
This week’s expert:
Dr Michael Wehrhahn, Clinical Microbiologist and Infectious Diseases Physician at Douglass Hanly Moir Pathology
Drawn from Dr David Lim’s interview with Dr Michael Wehrhahn on the Healthed Podcast, ‘Going Viral’.
• Symptomatic patients who test negative to SARS-CoV-2 after a nasopharyngeal PCR test can have a high level of confidence in that negative result. There is no need to repeat the COVID-19 testing in the vast majority of cases.
• Should the symptoms persist, it may be beneficial to consider doing a respiratory viral screen on the nasopharyngeal swab in order to provide the patient with an explanation for their symptoms. Currently 50% of people with RTI symptoms and a negative COVID-19 test, are testing positive for rhinovirus.
• If the patient has symptoms and has tested negative to SARS-CoV-2, and there is a high pre-test probability that they have COVID-19 (they have come from overseas or have had close contact with an infected person), repeat the PCR testing on another nasopharyngeal swab in 1-2 days.
• In Australia, all positive SARS-CoV-2 tests are double and triple checked with different assays. It is extremely unlikely that a positive result will prove to be a false positive. Nonetheless, this is a situation where serology is likely to prove useful. A positive serology test done a few weeks after the original positive PCR will confirm a COVID-19 infection.
• Asymptomatic patients are not ideal to swab for SARS-CoV-2, even if they are contacts of known cases. Even if they are incubating the disease, the viral load is unlikely to be sufficient to be detected through PCR testing. A higher yield occurs once symptoms develop.
• Current assays being used around the world for testing for acute infections are very accurate. The accuracy of reports of the number of cases in different countries differs, not by the quality of the testing but more by how extensive the testing is.
• Because of Australia’s extensive testing, we can be confident we are detecting over 90% of all cases of COVID-19. This also means that our case fatality rate, which is around 1%, is a true representation of the mortality associated with this disease – in a developed country. In countries where the case fatality is much higher eg the USA where the case fatality is closer to 5%, it is likely that the testing has not been as widespread and there remains a large population of affected but undiagnosed cases in the community.
• Many laboratories are currently evaluating new serology tests for SARS-CoV-2 that will have a high specificity for past COVID-19 infection. However, there is little role for clinicians to be requesting serology for COVID-19 at this stage.