Mpox Virus Colorized transmission electron micrograph of immature mpox virus particles (red with tan viral envelope) found within an infected VERO E6 cell (teal), cultured in the laboratory.
Image Credit: National Institute of Allergy and Infectious Diseases
Scientific Frontline: Extended "At a Glance" Summary: False-Positive Mpox Diagnoses
The Core Concept: A significant portion of positive mpox PCR tests may indicate environmental contamination rather than active viral infection.
Key Distinction/Mechanism: Researchers identified that true active infections cluster at low CT values (high viral load), while false positives cluster exactly at the detection threshold, indicating residual viral traces likely picked up from contaminated surfaces in treatment centers.
Origin/History: The study analyzed PCR results from four Congolese cities between April 2024 and April 2026, following the WHO's August 2024 declaration of the mpox upsurge in the Democratic Republic of the Congo as a Public Health Emergency of International Concern.
Major Frameworks/Components:
- PCR testing relies on CT values to determine viral load.
- Mpox viral DNA is highly stable and can persist for weeks on surfaces or in the air, creating a high risk of cross-contamination during patient sampling.
- Serological data supported the findings, with 89% of suspected false positives showing no trace of antibodies against the virus.
- Lowering the PCR positivity threshold was identified as a viable method to reduce false positives without significantly increasing false negatives.
Branch of Science: Virology, Epidemiology, Public Health.
Future Application: The findings strongly advocate for adjusting PCR positivity thresholds and improving the training of laboratory staff in low-resource settings to better identify testing anomalies and prevent misdiagnosis.
Why It Matters: False diagnoses lead to unnecessary overtreatment, hospitalization, and quarantine, unnecessarily exposing uninfected individuals to the virus in treatment centers while wasting critical public health resources.
An international team led by UNIGE has found that one-third of positive mpox PCR tests conducted in the Democratic Republic of the Congo (DRC) reflected environmental contamination of samples rather than genuine infection.
In August 2024, the WHO declared the mpox upsurge in the Democratic Republic of the Congo (DRC), and its spread to neighboring countries, a Public Health Emergency of International Concern. The emergence of a new strain and its rapid spread across eastern DRC, and later to other African countries and beyond, made reliable diagnosis essential for understanding the scale of the epidemic and guiding vaccination and other public health measures. A study led by the University of Geneva (UNIGE), the Institute of Tropical Medicine Antwerp (Belgium), and the National Institute of Biomedical Research (DRC) analyzed more than 2,700 PCR results for mpox in four Congolese cities—Goma, Kamituga, Kinshasa, and Uvira—between April 2024 and April 2026. The findings show that 35% of samples reported as PCR-positive may not have represented true infections but rather environmental contamination with mpox viral DNA. Published in The Lancet Infectious Diseases, the study has direct implications for outbreak surveillance and for the individuals falsely diagnosed.
Mpox is a viral disease characterized by fever and a distinctive rash that can cause painful and disfiguring skin lesions and, in some cases, serious or fatal complications. Fast, reliable case detection is essential for measuring the scale of an outbreak, targeting vaccination, adjusting public health measures, and slowing transmission—particularly in resource-limited settings.
PCR testing plays a central role in mpox diagnosis, as clinical symptoms are not specific to this virus. But when the same unusual pattern emerged in results from several different treatment centers, the team led by Andrew Azman, associate professor at the Institute of Global Health and the Center for Emerging Viral Diseases at the UNIGE Faculty of Medicine, took notice. Instead of a continuous distribution of CT values (the indicator reflecting the amount of virus detected in a sample—the lower the value, the higher the viral load), the results clustered sharply into two distinct groups.
"The first group corresponds to low values, consistent with a high viral load and therefore a genuine active infection," says Azman. "The second cluster sat just at the detection threshold, pointing to an extremely low viral load—atypical for a real infection, but typical of a residual viral trace. 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."
Contaminated Surfaces
To understand why, the team swabbed surfaces in the treatment areas of two care centers. Many of the surfaces tested positive for traces of the virus, while surfaces sampled outside the treatment areas came back negative. Mpox DNA is highly stable and can persist for several weeks on surfaces, or even remain in the air of treatment centers. This environmental contamination can then be picked up by mistake when sampling other patients, with no connection to an actual infection.
"We also looked at serological data for these individuals, where available," explains Megan O'Driscoll, the study's first author and a postdoctoral researcher at the Institute of Global Health. "Eighty-nine percent of the people that our model predicts had false-positive diagnoses showed no trace of antibodies against the virus." In a small, separate study, the team tested samples from patients with suspected mpox for multiple rash-causing viruses and found that most people had other viruses, like varicella (chickenpox) and measles, with few testing positive for mpox.
Adjusting the Positivity Threshold
These false diagnoses carry real consequences—for public health resource allocation, and for the people affected: unnecessary overtreatment, hospitalization, or quarantine at an mpox treatment center, along with the social stigma associated with the disease, and a very real risk of contracting an actual infection there.
"The simplest and fastest fix is to lower the PCR positivity threshold," Azman adds. "A lower threshold significantly reduces the number of false positives without meaningfully increasing false negatives—a proposal currently under discussion with decision-makers. In the long term, this shows that we need to improve the training of lab staff in low-resource settings like this, so they can spot these anomalies in real time, rather than after the acute emergency is over."
Funding: This work is part of the mandate of the Geneva Center for Emerging Viral Diseases, a WHO collaborating center for diagnostics, which supports several countries in improving their diagnostic practices.
Published in journal: The Lancet Infectious Diseases
Authors: Megan O'Driscoll, Elise De Vos, Levi Bugwaja, Isabel Brosius, Sabin Sabiti Nundu, Patrick Musole Bugeme, Justin Bengehya, Trust Faraja Mukika, Patrick Kazuba Bugale, Tony Wawina-Bokalanga, Eugene Bangwen, Sarah Houben, Fiston Isekusu Mpinda, Emmanuel Hasivirwe Vakaniaki, Erwan Piriou, Charles Tehoua, Fiston Nepa, Papy Munganga, Jean Claude Tshomba, Lorenzo Subissi, Grace Odia-Kadima, Meris Matondo-Kuamfumu, Antoine Nkuba-Ndaye, Jackie Knee, Jules Jackson, Thierry Kalonji, Elizabeth C. Lee, Noémie Lefrancq, Javier Perez-Saez, Salomon Mashupe Shangula, Krutika Kuppalli, Daniel Mukadi-Bamuleka, Espoir Bwenge Malembaka, Placide Mbala, Laurens Liesenborghs, and Andrew S. Azman
Source/Credit: Université de Genève
Edited by: Scientific Frontline
Reference Number: vi092226_01
