These limitations have stirred some criticism of PCR as a diagnostic tool.20 Often, if a patient has any detectable virus by PCR, they’re judged to be positive for coronavirus and are treated as if they’re infectious. Sometimes labs will adjust the cycle thresholds needed to call a test positive and exclude results that are reached only by repeatedly cycling the machines, say forty or more times.21 The challenge is that a high Ct value can mean that the patient has largely cleared the infection and is no longer contagious, but it can also mean that the person is newly infected, and the virus
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