Clinical Performance and Trends During the First Two Months of Monkeypox Virus PCR Testing at Two United States Reference Labs

Abstract

AbstractRecently, a sustained human-to-human outbreak of monkeypox virus (MPXV), a member of the Orthopoxvirus genus, which includes the etiologic agent of smallpox, has been documented in multiple non-endemic countries including the United States. Prior to June 2022, US testing was limited to public health labs and the Centers for Disease Control and Prevention. Following recognition of the scope of the outbreak, testing for MPXV has expanded into clinical laboratories. Here, we examine epidemiological characteristics, specimen collection practices, and cycle threshold (Ct) values for MPXV PCR tests performed at two reference laboratories.Results from both laboratories support public health data showing a high positivity rate in men (>30%) and those ages 30-49 (25-35%). The overall positivity rate decreased during the study period but remains elevated (∼20%). There was a significant difference in Ct values between laboratories (ARUP 23.86 vs. UW 25.40) and collection method (22.79 for dry swab vs. 24.44 for VTM). These viral load differences likely reflect slight differences in specimen processing. When multiple specimens were collected for a single individual, the overall result concordance rate was greater than 95%, with less than 1.5% of individuals having three or more tests receive a single positive result. As compared to the overall positive cohort, individuals three or more swabs and a single positive result had significantly higher Ct values (22.9 vs 35.0). These results provide an early snapshot of testing in the US during the monkeypox virus outbreak and support restricting the number of swabs collected per individual.