Analysis of cycle threshold values in SARS-CoV-2-PCR in a long-term study

BACKGROUND: Cycle threshold (Ct) values can be used in an attempt to semiquantify results in the qualitative real-time polymerase-chain-reaction (PCR) for the new coronavirus SARS-CoV-2. The significance of Ct values in epidemiological studies and large cohorts is still unclear. OBJECTIVE: To monitor Ct values in a long-term study and compare the results with demographic data of patients who tested positive for SARS-CoV-2 by real-time PCR. STUDY DESIGN: S gene SARS-CoV-2 Ct values were analyzed retrospectively from consecutive patients between March 15th to September 15th 2020 with special regard to age, gender, and in- or outpatient status. RESULTS: In total, 65,878 patients were tested, 1103 (1.7 %) of whom were positive for SARS-CoV-2. Twenty-six positive patients were excluded, because the respective PCR runs did not meet the stability requirements (Ct value of the positive controls between 26 and 29). Of the remaining 1077 patients, females (n = 566; 53 %) were significantly older than males (n = 511; 47 %) (50.9 versus 45.1 years; p = 0.006) and had slightly higher mean Ct values than males (25.4 vs. 24.8; p = 0.04). Patients in the age groups >80 years had significantly higher Ct values than the remaining age groups (p < 0.001). Children (0–19 years) showed Ct values in the range of those found in adults (25.2 vs. 25.1, p = 0.9). There were no statistically different Ct values between in- and outpatients (p = 0.1), however, SARS-CoV-2 positive inpatients were significantly older than outpatients (p < 0.0001). CONCLUSIONS: CT values are suitable for more detailed monitoring of the SARS-CoV-2 pandemic. Age is an important cofactor in SARS-CoV-2 positive patients and may have influence on Ct values in SARS-CoV-2-PCR.