08/23/2022: Lab Advisory: Mpox Virus Testing Considerations to Prevent False Positive Test Results

CDC's Laboratory Outreach Communication System (LOCS)

Audience: Clinical Laboratory Professionals

Level: Laboratory Advisory

To prevent false positive test results, CDC recommends that laboratory professionals perform repeat testing to verify positive diagnostic results for Orthopoxvirus or mpox virus DNA in specimens with high Cycle Threshold (Ct) values from persons who do not meet identified epidemiologic risk criteria for mpox.

Review laboratory test results (including raw data, PCR curves, etc.) carefully before you report results. Because molecular tests (e.g., real-time PCR tests) are very sensitive, cross-contamination is possible. If you obtain a high Ct value (generally ~34 or higher), CDC recommends to immediately re-extract and re-test to ensure there was no cross-contamination. CDC suggests this approach based on high Ct value alone, even in the absence of epidemiologic information. When possible, re-extract and re-test before you report results. Obtain approval from your CLIA director before you implement this recommendation in your laboratory.

Refer to the Health Alert Network (HAN) Health Update for more information on testing for mpox.

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Thank you,

The Laboratory Outreach Communication System

Laboratory Outreach Communication System (LOCS) | Division of Laboratory Systems (DLS)

Center for Surveillance, Epidemiology, and Laboratory Services (CSELS)

Centers for Disease Control and Prevention (CDC)

LOCS@cdc.gov