Reverse transcriptase-PCR detects SARS-CoV-2 RNA in a nasopharyngeal swab. The E gene Ct value is 18. A second confirmatory PCR targeting the N gene shows Ct 19. What does this low Ct value indicate clinically?
- A High viral load; patient is likely highly infectious ✓
- B Low viral load; patient is in the late recovery phase
- C Ct value does not correlate with viral load; it only confirms presence of virus
- D Ct <20 indicates reinfection rather than primary infection
Correct answer: A. High viral load; patient is likely highly infectious
Explanation
In RT-PCR, Ct (cycle threshold) value is inversely proportional to viral load: the fewer cycles needed to reach the threshold fluorescence, the higher the initial template (viral RNA) concentration. A Ct of 18-20 indicates a high viral load and correlates with greater infectiousness. Ct values >35-40 typically indicate very low or residual RNA (often non-infectious). Ct is used in infection control contexts to guide isolation decisions, and low Ct values early in illness indicate peak contagiousness.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.