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AACC Discourages Use of Cycle Threshold Values for SARS-CoV-2 Test Results

NEW YORK – The American Association for Clinical Chemistry released guidance recently advising against the use of cycle threshold values when managing COVID-19 patients with PCR tests.

Throughout the pandemic, healthcare providers and public health agencies have requested labs report the numerical Ct value in addition to the qualitative result when reporting a SARS-CoV-2 test. The Ct value is the number of cycles needed for an amplicon to become detectable above background, and lower Ct values are associated with a higher copy number in the sample tested.

However, AACC noted in its guidance that validation of a quantitative test is more complex and requires more rigorous testing to ensure test performance, such as determining the lower limit of quantification rather than the lower limit of detection.

Multiple calibrators are required to determine these values, but certified reference materials aren't yet incorporated into tests that have received Emergency Use Authorization from the US Food and Drug Administration. In addition, correlation of Ct values with viral load, viral burden, or infectivity hasn't been established for these tests, AACC said.

There are also multiple variables that can affect Ct value before, during, and after testing, such as patient preparation, biological variance, specimen type, gene targets, patient immunity, assay targets, and transport and storage.

AACC's guidance follows similar recommendations from both the Association for Molecular Pathology and the Infectious Diseases Society of America discouraging the use of Ct values for SARS-CoV-2 test results.

AACC added that for laboratories that do report Ct values, it suggests including a comment about the difficulties with using Ct values to guide patient management in the laboratory report. It also recommended labs consider assay specific Ct ranges and providing semi-quantitative results.

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