More than one in three people who tested positive for Mpox in several treatment centres in the Democratic Republic of the Congo (DRC) may not have been infected with the virus, according to a new study published in The Lancet Infectious Diseases.
Researchers say some of the false-positive results may have been caused by traces of Mpox virus DNA left on surfaces inside treatment centres and accidentally transferred to samples collected from patients.
The study, led by researchers from the University of Geneva, the Institute of Tropical Medicine Antwerp and the DRC’s National Institute of Biomedical Research, analysed more than 2,700 PCR test results collected between April 2024 and April 2026 from four locations: Goma, Kamituga, Kinshasa and Uvira.
Mpox is a viral disease that can cause fever, swollen lymph nodes and painful skin lesions. Because its symptoms can resemble other diseases such as chickenpox and measles, laboratory testing is important for confirming infections.
PCR tests detect genetic material from the virus. But the researchers found that some tests were detecting extremely small amounts of Mpox DNA that appeared to come from the environment rather than from an infected patient.
The warning sign was in the cycle threshold, or Ct, values produced by the tests. A low Ct value generally means that a sample contains a large amount of viral material, while a high Ct value means that only a small amount has been detected.
Andrew Azman, an associate professor at the University of Geneva, said researchers noticed two distinct groups in the test results.
“The first group corresponds to low values, consistent with a high viral load and therefore a genuine active infection,” Azman said.
The second group consisted of results close to the test’s detection limit.
“What’s striking is that this clear split between the two groups appeared almost identically across all four sites, which is unlikely to be a coincidence,” he said.
To investigate the possible source, the researchers collected swabs from surfaces in two Mpox treatment centres. Many surfaces inside the treatment areas contained traces of Mpox DNA, while surfaces outside those areas tested negative.
Mpox DNA is relatively stable and can remain on surfaces for weeks. This creates a possibility that viral material from contaminated furniture, equipment or other surfaces could accidentally get onto a patient’s sample during collection.
The researchers estimated that 35% of PCR results with Ct values below 40 may have been false positives, meaning they were more consistent with environmental contamination than an actual infection.
The problem appeared particularly significant among young children. Among children aged 0 to 4 who tested positive under the standard threshold, about 52% of results were estimated to be false positives.
The researchers also examined blood samples from some patients to look for antibodies produced after infection. Among people whom their model classified as likely false positives, 89% did not develop antibodies against mpox, providing additional evidence that many had not actually been infected.
Megan O’Driscoll, the study’s first author, said the antibody findings supported the researchers’ concerns.
“89% of the people that our model predicts had false-positive diagnoses showed no trace of antibodies against the virus,” she said.
False diagnoses can have serious consequences. People who are incorrectly diagnosed may experience stigma, unnecessary treatment or isolation. They may also be admitted to mpox treatment centres, potentially exposing them to people who genuinely have the infection.
The researchers say one practical solution is to adjust the threshold used to classify PCR tests as positive. Reducing the threshold from a Ct value below 40 to below 37 lowered the estimated false-positive rate from 35% to 18%. Reducing it further to below 34 brought the estimated false-positive rate down to about 5%, while retaining about 97% of genuine infections.
“The simplest and fastest fix is to lower the PCR positivity threshold,” Azman said. “A lower threshold significantly reduces the number of false positives without meaningfully increasing false negatives.”
The researchers also recommend stronger cleaning and decontamination of treatment areas, locally appropriate Ct thresholds and additional investigation of samples with very high Ct values.
The findings offer an important lesson for Mpox surveillance: detecting a small trace of viral DNA does not necessarily mean that a person is infected. As Mpox continues to circulate in the DRC and elsewhere, more accurate testing will be important for measuring the true scale of outbreaks.
Additional Information from Eureka
Image source