Swiss researchers find widespread mpox test contamination in DRC

Nearly 35% of more than 2,700 mpox samples may have carried contamination rather than infection, according to an international study led in part by the University of Geneva. The tests were collected between April 2024 and April 2026 in Goma, Kamituga, Kinshasa and Uvira, four locations central to the Democratic Republic of the Congo’s outbreak response.

The finding could force health authorities to reassess how they count cases and direct scarce resources. A positive molecular test normally triggers clinical follow-up, isolation advice and, in some circumstances, contact tracing. If viral DNA from a contaminated surface or clinical environment enters a specimen, those measures can reach people who were never infected.

The study, published in The Lancet Infectious Diseases, identified two distinct groups of positive results. One group showed higher viral levels, consistent with active infection. The other barely crossed the detection threshold, raising suspicion that the result reflected residual traces rather than virus inside the patient.

The researchers’ conclusion does not erase the DRC’s mpox outbreak. It points to a surveillance system that needs more precise laboratory safeguards, particularly where testing takes place under pressure.

Surface samples from two healthcare centres frequently tested positive for mpox DNA. The result offered a possible explanation for the low-level signals found in many patient tests. According to the University of Geneva, mpox DNA can remain on surfaces or in the air for several weeks, allowing traces to reach collection areas, equipment or specimens handled later.

The pattern mattered. Researchers saw one cluster of results with low cycle thresholds, indicating a larger amount of viral material, and another that only just met the laboratory’s detection limit. That second group was more consistent with residual environmental DNA than with a high viral load in a newly infected patient.

Contamination can occur without anyone deliberately mishandling a sample. Busy clinics may process many suspected cases in confined spaces, while staff move between examination, sampling and laboratory areas. The study highlights the need for strict separation between clean and contaminated zones, careful surface management and quality controls that can identify weak signals.

The researchers are working with the World Health Organization to improve detection methods in several countries. Their findings give laboratories a practical warning: a positive result close to the threshold deserves closer scrutiny and, where possible, confirmation through clinical and serological evidence.

Around 89% of people classified as false positives had no antibodies against mpox. The serological evidence strengthened the researchers’ assessment that these individuals had not been infected, even though their initial molecular tests detected viral DNA.

The team also examined suspected cases for other viruses that cause skin rashes. Most of the people tested carried other infections, while only a small proportion had mpox. That result matters because fever, lesions and painful rashes do not identify mpox on their own. Other pathogens can produce similar symptoms, particularly in communities where several infections circulate at the same time.

A laboratory result therefore sits within a wider diagnostic process. Clinicians need to consider symptoms, exposure history, the amount of viral material detected and, where appropriate, antibody results. Weak positive signals can become especially difficult to interpret when clinics face large numbers of patients and limited capacity for repeat testing.

The study does not establish that every disputed result was caused by contamination. It shows that environmental DNA and other infections must be considered when surveillance data appear inconsistent with clinical or immunological evidence. Better confirmation could give Congolese health authorities a clearer picture of where transmission is active and where resources are most urgently needed.