The World Health Organisation has updated its guidance on PCR tests to ensure they are used properly.
Target audience: laboratory professionals and users of IVDs.
Purpose of this notice: clarify information previously provided by WHO. This notice supersedes WHO Information Notice for In Vitro Diagnostic Medical Device (IVD) Users 2020/05 version 1, issued December 14th 2020.
Description of the problem: WHO requests users to follow the instructions for use (IFU) when interpreting results for specimens tested using PCR methodology.
Users of IVDs must read and follow the IFU carefully to determine if manual adjustment of the PCR positivity threshold is recommended by the manufacturer.
WHO guidance Diagnostic testing for SARS-CoV-2 states that careful interpretation of weak positive results is needed. 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. 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.
For months now, sceptics have been ridiculed for questioning the accuracy of the PCR test, referring to the risk of false positives when prevalence is low and urging the Government to carry out confirmatory second tests on those that test positive. This updated advice from the WHO is a vindication of our position. We look forward to Governments and health authorities bringing their practices into line with it.
Stop Press: George Michael writes that “False-Positives are Crushing the NHS“.
This analysis explores the very likely possibility that staff, patients, and the NHS as a whole are being mismanaged due to the significant proportion of positive COVID-19 cases in Pillar 1 that are false (where Pillar 1 represents the data from tests carried out in Public Health England (PHE) labs and NHS hospital settings).
Worth reading in full.