AMP urges caution in applying Ct values in clinical practice

April 22, 2021

The Association for Molecular Pathology (AMP) issued a joint statement with the Infectious Diseases Society of America (IDSA) urging caution in the use of SARS-CoV-2 PCR cycle threshold (Ct) values for clinical decision-making.

“This all may change in the future based on expanded development and standardization of test methods and more robust clinical data, but at the current time, routine use of Ct values to inform clinical decisions is not advised,” said Blake W. Buchan, PhD, D(ABMM), who is a member the AMP Clinical Practice Committee (Infectious Diseases Subdivision) and also Associate Professor in the Department of Pathology at the Medical College of Wisconsin.

Buchan’s comments were posted on AMP’s website. He notes that current real-time PCR tests for SARS-CoV-2 are designed as qualitative assays that generate a binary result of “positive” or “negative.”

“Some of the tests also generate and report a cycle threshold (Ct) value, referring to the number of PCR cycles required to amplify the targeted viral nucleic acid to a detectable level, Buchan wrote. However, he also said that “there are no commercially available molecular tests for SARS-CoV-2 that have data supporting an indication to report quantitative results, including Ct values.”

Many factors could impact the precision of the Ct values, he said, such as the use of different specimen collection devices, specimen types, nucleic acid extraction methods, genomic targets, and RT-PCR chemistries.

“Reporting Ct values for public health or epidemiologic studies may be important to the continued research and understanding of SARS-CoV-2 infection and transmission dynamics. However, caution is urged in reporting these values formally into the medical record,” he wrote.

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