WHO guidance Diagnostic testing for SARS-CoV-2 states that careful interpretation of weak positive results is needed (1). The cycle threshold (Ct) needed to detect virus is inversely proportional to the patient’s viral load. Where test results do not correspond with the clinical presentation, a new specimen should be taken and retested using the same or different NAT technology.
WHO reminds IVD users that disease prevalence alters the predictive value of test results; as disease prevalence decreases, the risk of false positive increases (2). This means that the probability that a person who has a positive result (SARS-CoV-2 detected) is truly infected with SARS-CoV-2 decreases as prevalence decreases, irrespective of the claimed specificity.
Most PCR assays are indicated as an aid for diagnosis, therefore, health care providers must consider any result in combination with timing of sampling, specimen type, assay specifics, clinical observations, patient history, confirmed status of any contacts, and epidemiological information.
The use of PCR testing has been criticised by many commentators as inappropriate for almost as long as the tests have been in use. The reality is that high numbers of cycles tend to give false positives. These tests also have no capacity to differentiate between active and inactive virus. It means that asymptomatic people are forced to quarantine when there is virtually no risk of them being infectious. That is equally true of the more transmissible versions of the virus currently spreading.Click HERE to subscribe to Fuller Treacy Money Back to top