Mpox rapid tests: useful when positive, unsafe when negative

Design

retrospective diagnostic accuracy study with masked operators, five antigen tests compared against a PCR reference on paired lesion swabs

Population

190 patients with suspected mpox in nine health zones of Kinshasa province, DR Congo, February to June 2025

Primary outcome

sensitivity and specificity of five antigen-based rapid diagnostic tests against PCR

Effect

best assay sensitivity 77.3% (95% CI 68.0-84.5) and specificity 93.5%; range across assays 39.2-77.3% sensitivity, 93.5-96.8% specificity

Much of Africa cannot deliver PCR-based mpox testing at the scale an outbreak requires, and antigen-based rapid tests have been proposed to fill the gap. This diagnostic accuracy study used lesion swabs from 190 patients with suspected mpox in nine health zones of Kinshasa province, taking six swabs per patient so that five commercial rapid tests could be compared against a PCR reference on the same lesion.

The best performer, from Guangdong Wesail Biotech, reached a sensitivity of 77.3% (95% CI 68.0-84.5) with specificity 93.5%. The Hangzhou Testsea assay was similar at 72.2% sensitivity. Beijing Hotgen managed 59.8% and Contipharma 50.5%, both with specificity above 95%. The NG Biotech assay had a sensitivity of 39.2% with specificity 96.8%.

Even the best of these misses roughly one infection in four, and the worst misses three in five. That is the number that should govern deployment: in a high-prevalence setting a positive result is informative and can support a diagnosis where PCR is unavailable, but a negative result cannot be used to release a patient from isolation or to close a contact-tracing chain. Used with that asymmetry made explicit, these tests extend reach; used as a rule-out, they would seed transmission. The same logic applies to any antigen test introduced under supply pressure, which is a lesson several countries have now learned twice.

• Use a positive antigen result to support diagnosis where PCR is unavailable in a high-prevalence setting
• Never use a negative antigen result to exclude mpox or to end isolation
• Sensitivity ranged from 39.2% to 77.3% across the five assays — the brand matters
• Where PCR is available, it remains the test; rapid tests extend reach rather than replace it
• Lesion swab quality drives antigen test performance; train collection alongside deployment

The statistics, in plain English

Sensitivity is the proportion of PCR-positive lesions the rapid test detects; specificity the proportion of PCR-negatives it correctly calls negative. High specificity with moderate sensitivity gives a high positive predictive value where disease is common — which is exactly the setting studied and not necessarily elsewhere, since predictive value falls as prevalence falls. Taking six swabs from the same patient reduces between-sample variation and makes this a fair head-to-head comparison. The design is retrospective and used stored swabs supplemented by PCR-negative samples from other studies, which can inflate specificity if those samples came from patients less likely to have skin lesions at all.

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