Pronounced Diurnal Pattern of Salivary C-Reactive Protein (CRP) With Modest Associations to Circulating CRP Levels

C-reactive protein (CRP), a humoral component of the innate immune system with important functions in host-defense, is extensively used as a sensitive biomarker of systemic inflammation. During inflammation, hepatocyte-derived CRP rises dramatically in the blood due to increased interleukin-6 (IL-6) levels. Reliable detection of CRP in saliva, instead of blood, would offer advantages regarding sampling procedure and availability but using saliva as a diagnostic body fluid comes with challenges. The aims of this study were to evaluate associations between salivary CRP, total protein levels in saliva and serum CRP. Furthermore, we examined associations with plasma IL-6, body mass index (BMI), tobacco smoking and age. Salivary CRP was investigated by ELISA in 107 middle-aged participants from the general population. We employed spectrophotometric determination of total protein levels. Correlation analyses were used for associations of salivary CRP with serum CRP (turbidimetry), plasma IL-6 (Luminex(R)), BMI and smoking habits. Salivary median CRP was 68% higher (p=0.009), and total protein levels were 167% higher (pFunding Agencies|Swedish Rheumatism Association [R-844801]; Region Ostergotland (ALF Grants) [LiO-932055]; Swedish Society of Medicine [SLS-590231]; King Gustaf Vs 80-year Anniversary foundation [FAI-2018-0504]; King Gustaf V and Queen Victorias Freemasons foundation