Strategic Retesting Identifies False Positive COVID-19 Test Results

Retesting positive COVID-19 PCR tests from asymptomatic, nonexposed persons and tests stored next to a specimen with high viral load identified false positives.

A recent study published in Pathology Research and Practice endeavored to detect false positive COVID-19 test results.

The investigators, from the University of Missouri School of Medicine, wanted to create a quality control procedure to identify false positive results from the reverse transcriptase polymerase chain reaction (RT-PCR) test.

The PCR test is considered reliable and is the most widely used method. However, it does produce a small number of false positives, often recognized in asymptomatic and nonexposed patients.

According to Lester Layfield, MD, professor of pathology and anatomical sciences at the University of Missouri School of Medicine and director of the Molecular Diagnostics Laboratory, false positives can lead to unnecessary quarantining and repeated testing of noninfected people, as well as waste valuable resources.

“False positive diagnoses have important implications for patient management. False positives may lead to inappropriate quarantine, delay of other necessary medical treatment or transfer to a COVID-19 ward,” Layfield said.

Layfield and fellow investigators began repeat testing all positive results from asymptomatic and unexposed patients. Additionally, they retested all specimens with positive results that were stored next to a specimen with a high viral load.

Beginning on September 2020, the team implemented the protocol for an 8-week period, during which 24717 RT-PCR tests were administered. Of these, 6251 were given to asymptomatic patients, 288 of which came back positive. Upon retesting, 20 of these proved to be false positives (6.9%).

Technologists’ errors, such as contamination or misplacing specimen in testing plate, and cross contamination from high viral load specimens in adjacent wells of tasting plates were common causes of false positive results.

“Retesting of positive results from asymptomatic individuals revealed some technologist errors but also contamination from positive specimens in adjacent specimen wells,” said Layfield. “This study should alert the laboratory testing community of the possibility of false positive COVID-19 tests.”