Direct quantification of creatinine in human urine by using isotope dilution extractive electrospray ionization tandem mass spectrometry.

Urinary creatinine (CRE) is an important biomarker of renal function. Fast and accurate quantification of CRE in human urine is required by clinical research. By using isotope dilution extractive electrospray ionization tandem mass spectrometry (EESI-MS/MS) a high throughput method for direct and accurate quantification of urinary CRE was developed in this study. Under optimized conditions, the method detection limit was lower than 50 μg L(-1). Over the concentration range investigated (0.05-10 mg L(-1)), the calibration curve was obtained with satisfactory linearity (R(2)=0.9861), and the relative standard deviation (RSD) values for CRE and isotope-labeled CRE (CRE-d3) were 7.1-11.8% (n=6) and 4.1-11.3% (n=6), respectively. The isotope dilution EESI-MS/MS method was validated by analyzing six human urine samples, and the results were comparable with the conventional spectrophotometric method (based on the Jaffe reaction). Recoveries for individual urine samples were 85-111% and less than 0.3 min was taken for each measurement, indicating that the present isotope dilution EESI-MS/MS method is a promising strategy for the fast and accurate quantification of urinary CRE in clinical laboratories.