Limitation of Interpreting a Single COVID Cycle Threshold Value

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Abstract The United States reported its first case of coronavirus disease 2019 (COVID-19) in January 2020. There was limited testing capacity at the time, and the causative agent severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was indeed extremely infectious. Testing, therefore, has become a vital part of controlling the COVID-19 pandemic. Nucleic acid amplification tests, such as polymerase chain reaction platform, a value called cycle threshold (Ct) is reported. This number represents the number of polymerase chain reaction test cycles it takes for the sample to become positive. It is inversely proportional to the amount of viral RNA present in the sample. Therefore, a high Ct value infers a low quantity of viral RNA, and conversely, a low Ct value implies high viral RNA, which in turn correlates with infectivity. When interpreted appropriately with the right clinical context, it could be used to assess the infectiousness of an individual. In September 2021, a 73-year-old man who previously completed a primary series of Moderna vaccine (2 injections) presented to the hospital with chest pain. Cardiac catheterization showed multivessel coronary artery disease and was deemed to have unstable angina. The plan was to undergo coronary aortic bypass graft surgery. Preoperative routine testing revealed a positive COVID-19 polymerase chain reaction test with a high Ct value. During the hospital stay, it was shown that he had a decreased Ct value and thus was determined to have an acute case of asymptomatic COVID-19 infection. His surgery was delayed according to the isolation protocol per Centers for Disease Control and Prevention for safety precautions. He eventually underwent coronary aortic bypass graft after completion of isolation and had an uneventful postoperative course. It was also documented that he had serological seroconversion consistent with acute COVID-19 infection. This case illustrates that reporting Ct values is useful in a certain clinical context, but clinicians need to be familiar with the transmissibility and history of SARS-CoV-2 infection as well as understanding the interpretation of positive tests. The appropriate clinical approach and management should be assessed based on exposure history, epidemiology, physical examination, radiographic assessment, and laboratory studies.