About a third of mpox PCR positives in DR Congo were likely environmental DNA, not infection

Design

Multi-site observational study with Bayesian latent class model

Population

2,724 suspected mpox cases at four DR Congo treatment centres

Primary outcome

Proportion of PCR positives attributable to environmental MPXV DNA

Effect

35% (CrI 31 to 39) at Ct below 40; 5% (3 to 6) at Ct below 34

A multi-site observational study in The Lancet Infectious Diseases (September 2026) analysed PCR cycle threshold (Ct) values from 2,724 suspected mpox cases at treatment centres in four DR Congo locations between 2024 and 2026, with surface sampling of clinics and laboratories at two sites. A Bayesian model separated true infections from positives consistent with environmental monkeypox virus DNA.

An estimated 35% (credible interval 31% to 39%) of PCR positives with Ct below 40 were likely false positives. Serology supported this: 89% of those classed as environmental had no evidence of orthopoxvirus infection. Lowering the cut-off to Ct below 37 reduced false positives to 18%, and below 34 to 5%, with only marginal loss of sensitivity.

The estimate is model-based and specific to high-transmission treatment centres, where viral DNA contaminates surfaces. It will be lower in low-prevalence settings. But the principle applies to any sensitive PCR for a DNA virus in a contaminated environment.

• Ask the laboratory for the Ct value on a positive mpox PCR, not just the result
• Treat a high-Ct positive with atypical illness as uncertain: repeat the swab and check epidemiological links
• Decontaminate sampling areas in clinics seeing many mpox cases
• Use local Ct cut-offs where the laboratory has validated them

Why it matters

Surveillance counts and case-finding in outbreak settings may be inflated by environmental contamination rather than infection.

Don't overread it

The false-positive estimate is model-derived and applies to high-transmission treatment centres, not low-prevalence clinics.

The statistics, in plain English

Ct is the number of PCR cycles needed to detect virus: a higher Ct means less DNA in the sample. A 35% false-positive estimate at Ct below 40 does not mean a third of all patients were misdiagnosed — the problem is concentrated in weak positives, which is why tightening the cut-off to 34 cut it to 5%.

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