Hardly any other medical procedure was as controversially discussed during the corona pandemic as the PCR test. Proponents saw this as the gold standard for detecting infections, while critics warned against automatically equating positive results with contagiousness.
During the pandemic, the total number of positive tests was used as a measure of the current infection rate and was used by politicians as a justification for numerous corona measures such as lockdowns and mask requirements.
The WHO also relied on positive PCR tests when it declared a public health emergency of international concern following the monkeypox outbreak in August 2024. The pathogen Mycoplasma pox (Mpox) spread from the Democratic Republic of Congo (DRC) to neighboring African countries.
Andrew Azman from the University of Geneva, together with colleagues from the Tropical Institute in Antwerp (Belgium) and the National Institute for Biomedical Research (DRK), has now taken another close look at the results of 2,700 positive PCR tests for Mpox. Tests from April 2024 to April 2026 were recorded.
In the specialist journal “The Lancet Infectious Diseases”, the researchers report that 35 percent – a good third – of the samples reported as PCR positive may not have actually represented an actual infection. Such a false positive result affects the surveillance of an outbreak and the scope of measures such as vaccinations, the researchers said.
The consequences for those falsely diagnosed as positive can also be serious: unnecessary treatments, hospital stays, quarantine and the stigmatization associated with the disease, right up to the risk of actually becoming infected with Mpox in a healthcare facility.
Further examinations showed that almost all of the people who had presumably tested false positive had no antibodies against the Mpox virus in their blood. Misdiagnoses and the associated unnecessary measures would put a strain on the already scarce resources of the Congolese health system, the researchers warn.
PCR – the abbreviation stands for polymerase chain reaction – is a laboratory method for detecting genetic material. Instead of looking for a needle in a haystack, the needle is multiplied millions of times until it is easy to recognize. The US biochemist Kary Mullis received the Nobel Prize in Chemistry in 1993 for developing this method.
PCR makes it possible to detect even tiny traces of virus genetic material. The more often the replication process - the so-called cycle threshold, CT for short - has to be repeated, the fewer virus particles are present in the sample; the number of cycles is called the CT value. Conversely, the lower the CT value, the higher the viral load.
And it is precisely this CT value that has the researchers suspicious. They did not find a continuous distribution of CT values in the samples examined - which would have been expected: At the beginning of the infection, the viral load is high and decreases towards the end of the acute phase. Instead, the results grouped clearly into two groups, as Azman explains in a statement from the University of Geneva.
“The first group includes low values, which indicates a high viral load and therefore an active infection,” said Azman. “The second group was right at the limit of detection, indicating an extremely low viral load – atypical for a real infection, but typical for a residual trace of virus.” The samples examined came from four Congolese cities – Goma, Kamituga, Kinshasa and Uvira.
“What is striking,” continues Azman, “is that this clear separation between the two groups occurred almost identically at all four locations, which is probably no coincidence.” The researchers therefore suspected contamination as a possible cause. They therefore took swabs from surfaces in the treatment areas of two care centers.
In fact, many of the surfaces showed traces of the Mpox virus. However, the samples taken outside the treatment rooms for comparison were negative. According to the researchers, the DNA of the Mpox virus is very stable and can survive for several weeks on surfaces and even in the air and in this way come into contact with patients who then test positive for Mpox.
Azman emphasized to WELT that the Mpox results come from an environment in the DRK in which intensive Mpox transmission and significant environmental contamination prevailed: "The figure of 'one third' should therefore not be understood as a general false positive rate for Mpox PCR tests and certainly not as a statement about PCR tests in general," warns Azman.
And further: "This environmental contamination was an important aspect of our study and plays a central role in the interpretation of our results. When we speak of false positive results, we mean that the detected DNA probably did not come from an active infection, but rather entered the sample from the environment due to the high sensitivity of the PCR."
A reliable diagnosis is therefore essential in order to be able to understand the true extent of an outbreak and respond appropriately. To achieve this, the laboratory staff would need to be better trained. The researchers concluded in their study that diagnostic procedures also need to be improved.
“The simplest and quickest solution is to lower the threshold for positive PCR tests,” said Azman. “A lower threshold significantly reduces the number of false positives without increasing the number of false negatives.”
The threshold value determines the CT value up to which a reliable result can still be expected. Beyond this threshold, a sample is considered negative. CT values between 10 and 15 indicate a high viral load and therefore an active infection.
The CT value was also an issue in Germany during the corona pandemic. Criticism was directed against assessing positive test results with high CT values as infectious. Epidemiologist John Ioannidis from Stanford University, among others, warned that the test results should be interpreted carefully. It was also discussed whether comprehensive PCR tests made sense at all for people without symptoms.
