Assessment of measurement of salivary urea by ATR-FTIR spectroscopy to screen for chronic kidney disease

Abstract

Stages of chronic kidney disease (CKD) are currently defined by estimated glomerular filtration rates (eGFR) and require measurement of serum creatinine concentrations. Previous studies have shown a good correlation between salivary and serum urea levels and the stage of CKD. However, quantitative salivary urea assays in current clinical use require costly and labour-intensive commercial kits which restricts the advantage of using saliva and limits wider applicability as a quick and easy means of assessing renal function. Attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy has been shown to provide a potentially straightforward, reagent-free method for the identification of a range of disease-related biomarkers and is in current clinical use for analyses of the chemical composition of kidney stones. We assessed the feasibility of ATR-FTIR spectroscopy as an alternative method to measure salivary urea in patients with different stages of CKD. The ATR-FTIR spectra of dried saliva samples from 6 healthy controls and 20 CKD patients (stages 1-5) were analysed to provide their urea concentrations. The lower limit of detection of salivary urea by the ATR-FTIR spectroscopy method was 1-2 mM, at the lower end of the clinically-relevant range. Statistically significant differences in salivary urea concentrations were demonstrated between healthy subjects (4.1±0.5 mM) and patients with CKD stages 3-5 (CKD stage 3: 6.8±0.7 mM; CKD stage 4: 9.1±1 mM; CKD stage 5: 14.8±1.6 mM). These salivary urea concentrations correlated well with serum urea levels in the same patients measured by an automated analyser (Spearman's rank correlation coefficient of 0.71; p<0.001). The ability of the method to detect and stage CKD was assessed from the sensitivity and specificity parameters of a receiver operating characteristics (ROC) curve analysis. This proof-of-concept study demonstrates that quantitation of salivary urea by ATR-FTIR spectroscopy could provide a viable tool for rapid and cost-effective diagnosis of stages 3-5 CKD.