WHO warns of 'false positives' in COVID tests, says some patients might not be 'truly infected'

Cycle threshold can help determine infection level, WHO says.

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A COVID-19 testing site in Los Angeles, CA

The World Health Organization is warning that widely deployed technology used to test for COVID-19 may, if interpreted incorrectly, lead at least some patients to believe they have the virus when in fact they are not "truly infected."

In a "medical product alert" issued last week, the WHO said that anyone "interpreting results for [COVID-19] specimens tested using [polymerase chain reaction] methodology" should be aware that "careful interpretation of weak positive results is needed."

The WHO's alert specifically addresses a controversial aspect of COVID-19 PCR tests: the "cycle threshold," or the number of amplification cycles the test must undergo before detecting a COVID-19 specimen.

If a test must go through a high number of cycles before detecting the COVID virus, it potentially indicates that the original viral specimen was weak and the patient from whom it came may not be infectious or even "truly infected," as the WHO put it in its announcement.

In its alert, the WHO said that the cycle threshold "is inversely proportional to the patient's viral load."

"[D]isease prevalence alters the predictive value of test results; as disease prevalence decreases, the risk of false positive increases," the alert states. "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."

The WHO's media relations arm did not address a question from Just the News about the prevalence of individuals who are not "truly infected" despite positive COVID-19 PCR tests.

The WHO said that after "thorough investigation" it "confirmed that [PCR] tests were not always being used appropriately and in accordance with the instructions provided by the manufacturer."

"In particular, users in laboratories encountered problems with these tests when they did not apply the recommended positivity threshold," the organization said. "This can result in either false negative results (if the threshold applied is lower) or false positives (if threshold is higher)."

The WHO said it issued a directive on this issue last month; the alert from last week was an update of that directive "with some language modifications to ensure greater clarity."

The reliability of PCR tests, as well as the transparency with which they are conducted, have been the source of controversy over the last year of the pandemic. 

Some critics have argued that medical officials should publicize the cycle thresholds of all COVID-19 tests so that the public can have a better grasp of the depth and severity of the pandemic in any given region. Federal coronavirus expert Anthony Fauci, meanwhile, last summer warned that PCR COVID-19 tests may in fact just pick up "dead" fragments of the virus, though he has in the intervening months argued for high testing levels as a way of fighting the virus. 

The issue has been examined by numerous media outlets several times during the course of the pandemic, including a report by the New York Times last August which reported that PCR tests are potentially "diagnosing huge numbers of people who may be carrying relatively insignificant amounts of the virus."

A November Just the News report on PCR tests, meanwhile, was partially censored by Facebook after what the company styles its "independent fact-checkers" argued that it was inaccurate to argue that some high-Ct-threshold COVID tests may constitute "false positives" — the very language the World Health Organization uses to make the same assertion in both its recent alert and its communication with Just the News.