Day-by-Day Distribution of SARS-CoV-2 RT-PCR Cycle Threshold Values in Outpatient Care: Associations with Symptom Onset and Fever Severity

Background/Objectives: Cycle threshold (Ct) values from SARS-CoV-2 RT-PCR are widely reported in clinical practice, yet their interpretation in outpatient settings remains challenging due to substantial temporal and clinical variability. This study aimed to characterize day-by-day Ct distributions after symptom onset and to evaluate how symptom timing and fever severity inform diagnostic interpretation in primary care. Methods: We conducted a single-center retrospective study of 906 outpatients with COVID-19 who underwent saliva RT-PCR testing (January 2022–April 2023). Ct values were summarized according to days since symptom onset (Day 0–14). Peak self-reported temperature was categorized into 1 °C strata (36–40 °C), with temperature analyses restricted to patients tested on Day 0. Spearman’s correlation and multivariable linear regression with 95% confidence intervals were used to assess associations. Results: Ct values increased with longer intervals from symptom onset but demonstrated substantial variability within each day (Spearman’s ρ = 0.166, p < 0.001). On Day 0, higher temperature strata were associated with lower Ct values (p = 0.018). In multivariable analysis, days since onset, temperature category, sex, and age group were independently associated with Ct values, whereas vaccination doses and comorbidities were not. Conclusions: Incorporating symptom onset timing and fever severity may support more nuanced, context-based interpretation of Ct values in primary care, rather than reliance on isolated thresholds.