In real-time PCR, the cycle threshold (Ct) value of a respiratory sample for SARS-CoV-2 is 38. The laboratory's validated cut-off for positivity is Ct ≤40. The clinical team asks about infectivity. The most accurate statement is:
- A Any PCR-positive result indicates the patient is infectious and must be isolated indefinitely
- B Ct value is independent of viral load and should not guide infection control decisions
- C High Ct values (>33) correlate with very low or absent viable virus and likely minimal infectivity ✓
- D A Ct of 38 is equivalent in infectivity to a Ct of 20
Correct answer: C. High Ct values (>33) correlate with very low or absent viable virus and likely minimal infectivity
Explanation
Ct values inversely correlate with viral load — each 3.3 Ct increase represents approximately a 10-fold decrease in viral copies. Studies have demonstrated that SARS-CoV-2 cultures are rarely successful from samples with Ct >33–35, suggesting minimal viable virus. High Ct results late in illness likely represent RNA fragments from resolved infection. While Ct is not the sole determinant of infectivity, it is a useful guide for infection control in combination with clinical context and symptom duration.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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Written and medically reviewed by the StethoPrep medical team.