Salivary C-reactive protein exhibits a diurnal pattern and relates to biobehavioral health in military men.

C-reactive protein is a systemic inflammatory biomarker that is positively associated with the development of disease. Salivary C-reactive protein (sCRP) has previously been reported to have a diurnal rhythm with higher levels upon awakening and lower levels thereafter. The aims of this study were to evaluate the stability of sCRP across two days, characterize the daily sCRP pattern, compute morning sCRP parameters, and evaluate associations with biobehavioral health in US Navy Explosive Ordnance Disposal (EOD) technicians. Seventy male EOD technicians (age = 34.9 ± 6.5 years) participated in this study, which included a tablet-based survey, measures of health and fitness, and saliva collection. In a free-living setting, participants self-collected saliva on 2 consecutive days at WAKE, WAKE+30, WAKE+60, 4p.m., and 9p.m., for a total of 10 samples. Parameters (e.g., area under the curve) were computed to characterize the morning sCRP magnitude and pattern. Pearson product-moment correlation analyses were used to assess the stability of sCRP samples and parameters across the study period and to examine associations with biobehavioral health. Average sCRP concentrations for the 2-day period were evaluated using an analysis of variance with repeated measures. The stabilities between corresponding time points on Days 1 and 2 were very high (rs = 0.87-.94, all ps ≤ 0.001). sCRP concentrations were highest at WAKE, decreased by 73.6 % at WAKE+30, and then plateaued for the rest of the day. Parameter stabilities were good to excellent (rs = 0.77-.98, all ps ≤ 0.001). We also observed associations between sCRP parameters, self-reported health behaviors, and objective measures of health and fitness. In this study of a military population, we characterized sCRP as diurnal with robust stability across 2 consecutive days, which demonstrates the feasibility of sCRP as a biomarker. These results have significant implications for study methodology and for using sCRP as a marker of dysfunction or disease.