Environmental DNA may be inflating mpox PCR positives

Design

Multi-site observational study with Bayesian latent-class modelling

Population

2,724 people with mpox-compatible illness in four DR Congo locations

Primary outcome

Proportion of qPCR positives attributable to environmental DNA

Effect

35% (95% CrI 31-39) of positives at Ct<40 likely false; falling to 5% at Ct<34

Highly sensitive qPCR can pick up environmental viral DNA, not just infection. This multi-site study in DR Congo examined cycle threshold values from 2,724 people with mpox-compatible illness during a widespread outbreak, with surface sampling of clinics and a Bayesian model validated against serology.

The estimate is striking: about 35% of qPCR positives with a cycle threshold below 40 were consistent with environmental monkeypox DNA rather than true infection. Serology backed this, with most inferred false positives showing no orthopoxvirus antibodies. Tightening the diagnostic cutoff to a threshold below 37 cut the false-positive rate to 18%, and below 34 to 5%, with only marginal losses in sensitivity.

The practical message for diagnostic stewardship is that a positive mpox PCR with a high cycle threshold, in a high-transmission setting, should be interpreted with caution: strengthen decontamination, set locally appropriate cutoffs, and consider repeat testing or clinical and serological correlation for borderline results.

• Multi-site study of 2,724 suspected mpox cases in DR Congo with environmental surface sampling.
• About 35% of qPCR positives with cycle threshold below 40 were consistent with environmental DNA.
• Most inferred false positives had no serological evidence of orthopoxvirus infection.
• A cutoff below 37 cut false positives to 18%, and below 34 to 5%, with little sensitivity loss.
• Treat a high cycle-threshold positive with caution and consider repeat or serological testing.

Why it matters

It challenges the assumption that a positive mpox PCR always means active infection, with direct consequences for case counts and isolation.

Don't overread it

This is a modelled estimate from specific outbreak settings; it does not mean PCR is unreliable generally, nor give a single cutoff for all laboratories.

The statistics, in plain English

A 35% false-positive estimate applies to weak positives (high cycle threshold), where little target DNA is present, exactly where environmental contamination competes with true signal. Tightening the cutoff trades a small amount of sensitivity for a large gain in specificity.

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