Association Between Symptom Onset, Body Temperature, and SARS-CoV-2 RT-PCR Cycle Threshold Values: A Retrospective Analysis of 906 Outpatient COVID-19 Patients

Objective: Ct (cycle threshold) values from real-time PCR testing reflect SARS-CoV-2 viral load and are used to estimate infectivity and timing of infection. However, few studies in primary care settings have examined Ct values in relation to clinical findings—such as fever—on a day-by-day basis. This study analyzed the association between days since symptom onset and Ct values, and compared Ct values across temperature ranges on the day of symptom onset in outpatients with COVID-19. Methods: We conducted a retrospective observational study of 901 COVID-19 outpatients diagnosed via real-time PCR in our fever clinic from January 2022 to April 2023. We calculated days from symptom onset to specimen collection and analyzed daily Ct value trends from Day 0 to Day 14. We compared Ct values by temperature category (36°C–40°C) on Day 0. We also performed multivariable linear regression analyses with Ct value and temperature category as outcomes, adjusting for days since onset, temperature category, sex, age group, vaccination count, and comorbidities. Results: Ct values were lowest on symptom onset day and significantly increased over time (Spearman ρ = 0.59, p < 0.001). On Day 0, higher temperature ranges (38°C–39°C) were associated with significantly lower Ct values, indicating higher viral loads (p < 0.001). Multivariable analysis showed that days since onset (β = 0.683, p < 0.001), temperature category (β = − 0.348, p = 0.018), male sex (β = − 1.153, p < 0.001), and age (β = − 0.040, p < 0.001) were significantly associated with Ct values. Vaccination count and comorbidities were not significant. Conclusions: This study demonstrates that lower Ct values on symptom onset day correlate with higher body temperature, supporting the notion that viral load peaks early in infection. Ct values are influenced by multiple clinical factors including time since onset, temperature, age, and sex. Quantitative associations between fever and Ct values may aid in early infectivity assessments in clinical practice.