(A) Histogram of cAMP levels detected in the blood of RCC study participants without a known history of asthma or other lung diseases (n = 273) and patients with asthma in the SARP-3 (n = 87). For each sample, cAMP levels (pmol per 60 μl serum) were measured in duplicate by using the cAMP-Screen System ELISA kit (Applied Biosystems) and presented as mean ± SD. (B) Serum cAMP levels by asthma severity. Asthma severity was defined as “nonsevere” (n = 48) and “severe” (n = 39) according to European Respiratory Society/American Thoracic Society criteria. Linear regression models were used with age and sex as covariates across clinical groups. To satisfy the normalization assumption necessary for linear regression testing, cAMP levels underwent log transformation and the analysis was conducted using R version 4.4.1.