Assessment of salivary urea as a less invasive alternative to serum determinations

Objective. Experimental studies describe how urea is excreted through salivary glands and correlates with serum levels independently of salivary flow rate. This study confirms that salivary urea (SaU) is a reliable biomarker of uraemic state. In order to validate the SaU methodology, the following factors were taken into account: the independence of urea levels from saliva flow rate in healthy subjects and patients with chronic renal failure and the agreement between SaU and serum urea (U) levels in the entire population. In addition, reference intervals and cut-off values for SaU and U were established. Material and methods. Urea levels were determined in 268 matched whole saliva (SaU) and serum (U) samples obtained simultaneously from 78 healthy individuals and 154 patients with chronic renal failure. A serum enzymic colorimetric assay was adapted to SaU determinations. Results. SaU was independent of salivary flow rate. The agreement between SaU and U was confirmed by Bland-Altman analysis with a significant correlation between them (r=0.91, p=0.0001). The reference interval of SaU ranged from 1.66 to 7.5 mM. The cut-off values for SaU and U were 7.5 mM and 8.2 mM, respectively (sensitivity and specificity 100 % for both). Conclusions. SaU testing is harmless and useful for ruling out azotemic states in outpatients. Our results support the inclusion of SaU as a diagnostic test in the clinical laboratory.