Drop Date: April 2011

In This Drop: Salivary Bioscience and Inflammation

Bio-medical researchers routinely measure biomarkers such as cytokines (IL-6, IL-1ß, TNF-α) and C-reactive protein (CRP) in blood in order to assess the presence of inflammation. In order to avoid the cost, inconvenience, discomfort, and bio-hazard risk associated with blood drawing, however, researchers have more recently been exploring the utility of inflammatory markers in saliva.

Studies involving salivary IL-6, IL-1ß, and TNF-α have been appearing with increasing frequency, and interesting associations to psychological and physiological stress have been observed, along with associations to local inflammatory conditions such as periodontitis, diseases of the salivary glands, and oral cancer. The relationships of salivary levels of these cytokines to blood levels are still not well understood, however, and initial studies have suggested there is only modest correlation between the two fluids.

In the case of CRP, there is encouraging research that suggests salivary CRP may be more strongly associated with serum CRP than is true for other inflammatory markers. CRP, which originates largely from the liver, is too large a molecule to pass from the circulation into the salivary glands by diffusion or ultrafiltration as do many other salivary analytes, and it is believed that small amounts of CRP enter whole saliva mainly as a component of the gingival crevicular fluid (GCF) that flows out from the junctions between the teeth and gums. Whole saliva is much easier to collect than GCF, which could make saliva the oral fluid of choice for routine assessment of CRP in clinical or field studies.

As with the cytokines, however, the question of local production of CRP in the oral cavity must be considered, since it could potentially obscure the relationship between blood and salivary levels and the interpretation of salivary CRP in relation to other analytes of interest. In this issue of the Spit Report, we present several recent papers that examine the relationship between serum and salivary CRP, including a very recent study that is the first to report a moderately strong correlation between serum and salivary levels of CRP in humans.

Featured Article

Validation of a High-Sensitivity Assay for C-Reactive Protein in Human Saliva.

Ouellet-Morin, I., Danese, A., Williams, B., & Arseneault, L. (2011). Brain Behav Immun, 25(4), 640-46.

With the recent introduction of a number of high-sensitivity immunoassays that can measure CRP in the range of values normally found in saliva, researchers have begun to evaluate salivary CRP for its utility as a non-invasive measure of systemic inflammation. This study is one of the first conducted with humans. Blood and unstimulated passive drool samples were collected from 61 healthy adult volunteers in order to investigate the correlation of salivary CRP with serum CRP. In addition, the study examined associations of salivary CRP with known correlates of systemic inflammation (serum IL-6, BMI, and smoking); stability of CRP in saliva was also studied.

Salivary CRP values – but not serum CRP values – for ten of the participants exceeded 2 standard deviations from the mean reported in healthy adults by the assay manufacturer. These high values were not found to be related to saliva flow rate, saliva pH, blood contamination, health conditions, or medications. It was determined that these high values were not caused by poor precision of the assay for samples with CRP values beyond the dynamic range of the assay. Consequently, the high CRP values for these individuals were not explainable, and they were excluded from the analysis. The authors speculate that local inflammatory processes in the mouth may have led to high values in these individuals. They suggest that future studies should therefore include a complete oral health examination as part of the participants’ health assessment.

As expected, the saliva-to-serum ratio was low (1:1633). This finding is consistent with belief that CRP has a restrictive pathway of entry into saliva mainly through gingival crevicular fluid. The correlation between serum and salivary CRP was found to be moderately strong, r = 0.72. The association did not vary by age or sex, but the prediction of serum CRP levels from salivary levels was more accurate at higher serum mean levels. Importantly, analysis of the data found that salivary CRP was able to predict accurately 89% of the participants with high serum CRP levels (cutoff > 3 mg/L), which is considered a good measure of discrimination. Salivary CRP was also found to be associated with elevated serum IL-6, high BMI, and smoking. Examination of the effect of different storage conditions found that salivary CRP is stable at room temperature for up to 8 hours. Freezing on the day of collection is advised, however, to avoid degradation that occurs under longer term storage.

In summary, this study provides important initial evidence suggesting that salivary CRP may provide a valid prediction of serum CRP. This finding of a moderately strong correlation between salivary and serum CRP in humans agrees with strong saliva-serum CRP correlations that have been reported recently in animal studies. Additional human studies are needed, however, in order to confirm the serum-saliva correlation found here in a broader range of populations with expected high or low CRP levels. The authors discuss, for example, another recent study involving human participants that found no correlation between serum and salivary CRP (Dillon, et al. 2010).

Research Area: Health & Inflammation
Focus: Inflammation & Immune Function