Measurement of precipitated calcium citrate in 24-hour urine samples is clinically useful for ruling out calcium oxalate urolithiasis.

Objective: Combining calcium with citrate clinically prevents the formation of calcium oxalate (CaOx) stones. In this study, we developed a new method for measuring calcium citrate (CalCit) precipitated out of urine samples. Method: CalCit in urine was formed by challenging with excessive calcium and precipitated out by ethanol. The precipitated CalCit (pCalCit) was redissolved and measured by an indicator displacement assay (IDA) using murexide as an indicator. This method is called the murexide-based IDA for measurement of precipitated CalCit (miDA-pCalCit). Results: Urea, creatinine, ammonia, sulfate, carbonate, uric acid, oxalate, albumin, and bilirubin did not interfere with the pCalCit measurement. The limit of detection and limit of quantitation were 80 and 400 μM, respectively. The miDA-pCalCit test was clinically validated in 24-hour urine samples obtained from CaOx stone patients and nonstone forming (NSF) subjects. The urinary pCalCit levels of CaOx stone patients were significantly lower than NSF subjects. Receiver operating characteristic analysis revealed an area under curve of 0.823 (95% confidence interval [CI]: 0.752-0.893) for separating CaOx stone patients from NSF subjects. At the cutoff of 632 µM, the test provided sensitivity, specificity, positive predictive value, negative predictive value, positive likelihood ratio, negative likelihood ratio (LH-), and accuracy of 84.4%, 70.1%, 51.4%, 92.5%, 2.9, 0.2, and 74.3%, respectively. Conclusions: We successfully established the miDA-pCalCit test for measuring pCalCit in 24-hour urine samples. The urinary pCalCit levels in CaOx stone patients decreased significantly compared with NSF individuals. With an LH- of 0.2, the negative test result could be clinically helpful for ruling out CaOx urolithiasis.