About 35% of mpox PCR results reported as positive in four cities of the Democratic Republic of the Congo (DRC) may not reflect real infections, according to a study published on September 22, 2026, in The Lancet Infectious Diseases. The researchers conclude that many of these results were caused by mpox viral DNA contaminating the environment of treatment centres and laboratories, rather than by the virus in the patients themselves.
The study
The work was led by the University of Geneva (UNIGE) and its Geneva Centre for Emerging Viral Diseases, together with the Institute of Tropical Medicine in Antwerp, Belgium, and the National Institute of Biomedical Research (INRB) in the DRC. First author Megan O’Driscoll, a postdoctoral researcher at UNIGE, and colleagues analysed more than 2,700 mpox PCR results from Goma, Kamituga, Kinshasa, and Uvira, collected between April 2024 and April 2026.
Two groups of Ct values
The key observation came from the cycle threshold (Ct) values of the positive samples. Instead of a single continuous range, the values fell into two distinct groups. Low Ct values, which indicate a large amount of viral DNA, corresponded to genuine infections. High Ct values, which indicate only traces of viral DNA, were consistent with residual contamination.
Several lines of evidence supported this interpretation. Swabs taken from surfaces in treatment centres detected mpox DNA in the environment. Among the people whose results the researchers’ model classified as likely false positives, 89% had no antibodies against the virus. When samples were tested for other viruses, some patients turned out to have chickenpox (varicella) or measles rather than mpox.
Consequences for patients and surveillance
A false-positive mpox result can lead to unnecessary treatment, hospitalisation, or isolation in an mpox treatment centre, where the person may then be exposed to real infection. At population level, false positives inflate case counts, which can distort the picture of an outbreak and affect decisions such as where to send vaccines.
A simple fix
The authors recommend adjusting how PCR results are interpreted. “The simplest and fastest fix is to lower the PCR positivity threshold,” said Andrew Azman, Associate Professor at UNIGE. In practice, this means applying a stricter Ct cut-off, so that samples with only trace amounts of viral DNA are no longer reported as positive. They also call for more training of laboratory staff in resource-limited settings, including measures to prevent and detect contamination.
Lessons for PCR diagnostics
The study does not question PCR as a method. Real-time PCR remains the reference test for confirming mpox. It does show that the sensitivity of PCR also makes it vulnerable to contamination, especially in busy treatment centres and laboratories working under outbreak pressure. Validated Ct cut-offs, negative controls in every run, separation of pre- and post-amplification areas, and routine environmental monitoring all help ensure that a positive result reflects an infection.