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An international team led by UNIGE reveals that a third of the positive PCR tests for mox carried out in the Democratic Republic of Congo (DRC) reflected contamination of samples by the environment rather than a true infection.
The PCR test plays a central role in the diagnosis of medullary angina. © Adobe Stock
In August 2024, the WHO declared the mpox epidemic in the Democratic Republic of Congo (DRC), and subsequently in neighboring countries, a Public Health Emergency of International Concern. The rapid spread of a new strain from eastern DRC to other countries made reliable diagnostics essential to assess the scale of the epidemic and guide the response. A study conducted by the University of Geneva (UNIGE), the Institute of Tropical Medicine in Antwerp (Belgium), and the National Institute for Biomedical Research (DRC) analyzed more than 2,700 PCR test results for mpox in four Congolese cities—Goma, Kamituga, Kinshasa, and Uvira—between April 2024 and April 2026. Nearly 35% of the samples that tested positive were found not to correspond to actual infections, but rather to environmental contamination by mpox viral DNA. Published in The Lancet Infectious Diseases , this study has concrete consequences, both for epidemiological surveillance and for people who are wrongly diagnosed.
Pox is a viral illness characterized by fever and a distinctive rash that can cause painful and disfiguring skin lesions and, in some cases, serious or fatal complications. Rapid and reliable case detection is essential for assessing the scale of an outbreak, targeting vaccination, adjusting public health measures, and slowing transmission, especially in resource-constrained settings.
PCR testing plays a central role in diagnosing pox, as its clinical symptoms are nonspecific. But when the same unusual pattern appeared 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, was intrigued. 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 clearly fell into two distinct groups.
Contaminated surfaces
To understand why, the team collected samples from surfaces in the treatment areas of two healthcare facilities. Many surfaces tested positive for traces of the virus, unlike surfaces outside the treatment areas. Because mycobacterium tuberculosis DNA is very stable and can persist for several weeks on surfaces, or even remain present in the air of treatment facilities, this environmental contamination was likely detected by mistake in subsequent sampling, unrelated to any actual infection.
“We also looked at the available serological data,” explains Megan O’Driscoll, the study’s lead author and a postdoctoral researcher at the Institute for Global Health. “About 89% of the people our model identified as false positives actually had no trace of antibodies against the virus.”
In a separate, smaller study, the team tested samples from patients suspected of having mycobacterium tuberculosis for several other viruses that cause rashes. Most individuals were carriers of other viruses, such as chickenpox or measles; only a few were actually positive for mPox.
Adjust the positivity threshold
These false diagnoses have very real consequences for the allocation of public health resources, but also for the people concerned: unnecessary treatments, hospitalization or quarantine in a treatment center for tuberculosis, not to mention the social stigma associated with the illness and the very real risk of contracting a genuine infection there.
"The simplest and quickest solution is to lower the PCR positivity threshold," says Andrew Azman. "A lower threshold considerably reduces the number of false positives without significantly increasing the number of false negatives, a proposal currently being studied with the relevant authorities. In the longer term, this also shows that it is necessary to improve the training of laboratory staff in resource-limited settings like this one, in order to detect these anomalies in real time, rather than once the acute emergency has passed."
This study is part of the mandate of the Geneva Centre for Emerging Viral Diseases, a WHO Collaborating Centre for Diagnostics, which supports several countries in improving their diagnostic practices.
Contact
Andrew Azman,
Associate Professor,
Institute of Global Health,
Geneva Centre for Emerging Viral Diseases,
Faculty of Medicine
, University of Geneva,
+41 22 379 04 63,
Andrew.Azman@unige.ch
This research is published in
Lancet Infectious Diseases
DOI: 10.1016/S1473-3099(26)00411-1
------------------------------------------------------
See also:
The risk of mpox false positive results in high transmission settings: evidence from a multi-site observational study in DR Congo
...https://flutrackers.com/threads/the-lancet-the-risk-of-mpox-false-positive-results-in-high-transmission-settings-evidence-from-a-multi-site-observational-study-in-dr-congo.1040920/
Mpox: More than a third of positive tests are believed to be false diagnoses
Published on September 22, 2026An international team led by UNIGE reveals that a third of the positive PCR tests for mox carried out in the Democratic Republic of Congo (DRC) reflected contamination of samples by the environment rather than a true infection.
In August 2024, the WHO declared the mpox epidemic in the Democratic Republic of Congo (DRC), and subsequently in neighboring countries, a Public Health Emergency of International Concern. The rapid spread of a new strain from eastern DRC to other countries made reliable diagnostics essential to assess the scale of the epidemic and guide the response. A study conducted by the University of Geneva (UNIGE), the Institute of Tropical Medicine in Antwerp (Belgium), and the National Institute for Biomedical Research (DRC) analyzed more than 2,700 PCR test results for mpox in four Congolese cities—Goma, Kamituga, Kinshasa, and Uvira—between April 2024 and April 2026. Nearly 35% of the samples that tested positive were found not to correspond to actual infections, but rather to environmental contamination by mpox viral DNA. Published in The Lancet Infectious Diseases , this study has concrete consequences, both for epidemiological surveillance and for people who are wrongly diagnosed.
Pox is a viral illness characterized by fever and a distinctive rash that can cause painful and disfiguring skin lesions and, in some cases, serious or fatal complications. Rapid and reliable case detection is essential for assessing the scale of an outbreak, targeting vaccination, adjusting public health measures, and slowing transmission, especially in resource-constrained settings.
PCR testing plays a central role in diagnosing pox, as its clinical symptoms are nonspecific. But when the same unusual pattern appeared 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, was intrigued. 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 clearly fell into two distinct groups.
“The first group corresponded to low values, consistent with a high viral load and therefore with a real active infection,” the researcher explains. “The second group was right at the detection threshold, indicating an extremely low viral load—atypical for a real infection, but typical of a residual viral trace. What is striking is that this clear separation between the two groups appeared almost identically at all four sites, which hardly seems to be a coincidence.”
Contaminated surfaces
To understand why, the team collected samples from surfaces in the treatment areas of two healthcare facilities. Many surfaces tested positive for traces of the virus, unlike surfaces outside the treatment areas. Because mycobacterium tuberculosis DNA is very stable and can persist for several weeks on surfaces, or even remain present in the air of treatment facilities, this environmental contamination was likely detected by mistake in subsequent sampling, unrelated to any actual infection.
“We also looked at the available serological data,” explains Megan O’Driscoll, the study’s lead author and a postdoctoral researcher at the Institute for Global Health. “About 89% of the people our model identified as false positives actually had no trace of antibodies against the virus.”
In a separate, smaller study, the team tested samples from patients suspected of having mycobacterium tuberculosis for several other viruses that cause rashes. Most individuals were carriers of other viruses, such as chickenpox or measles; only a few were actually positive for mPox.
Adjust the positivity threshold
These false diagnoses have very real consequences for the allocation of public health resources, but also for the people concerned: unnecessary treatments, hospitalization or quarantine in a treatment center for tuberculosis, not to mention the social stigma associated with the illness and the very real risk of contracting a genuine infection there.
"The simplest and quickest solution is to lower the PCR positivity threshold," says Andrew Azman. "A lower threshold considerably reduces the number of false positives without significantly increasing the number of false negatives, a proposal currently being studied with the relevant authorities. In the longer term, this also shows that it is necessary to improve the training of laboratory staff in resource-limited settings like this one, in order to detect these anomalies in real time, rather than once the acute emergency has passed."
This study is part of the mandate of the Geneva Centre for Emerging Viral Diseases, a WHO Collaborating Centre for Diagnostics, which supports several countries in improving their diagnostic practices.
Contact
Andrew Azman,
Associate Professor,
Institute of Global Health,
Geneva Centre for Emerging Viral Diseases,
Faculty of Medicine
, University of Geneva,
+41 22 379 04 63,
Andrew.Azman@unige.ch
This research is published in
Lancet Infectious Diseases
DOI: 10.1016/S1473-3099(26)00411-1
------------------------------------------------------
See also:
The risk of mpox false positive results in high transmission settings: evidence from a multi-site observational study in DR Congo
...https://flutrackers.com/threads/the-lancet-the-risk-of-mpox-false-positive-results-in-high-transmission-settings-evidence-from-a-multi-site-observational-study-in-dr-congo.1040920/