Development of the LC-MS/MS method for determining the p-cresol level in plasma.

p-Cresol is a protein-bound uremic retention solute that originates in the intestine through bacterial metabolism and accumulates throughout the body in case of kidney failure. To date, there has been no method to analyze unconjugated p-cresol concentration in the blood with a limit of detection lower than 75 pg. Thus, the aim of this study was to develop and validate a novel liquid chromatography-tandem mass spectrometry method for the determination of unconjugated p-cresol in plasma with a lower detection limit than what has been determined using previously described methods. Sample preparation included derivatization of p-cresol with dansyl chloride (derivatization reagent) showed to be a better approach to analyze the compound. The method optimization involved various pH, time of the reaction, and concentration of derivatization reagent. The validation process was performed according to the procedures prescribed by the European Medicines Agency. All analyzed validation criteria were fulfilled. The novel validated method was applied to compare the level of p-cresol in patients with chronic renal failure before and after dialysis (n = 24). Additionally, the concentration of p-cresol was determined in patients with multiple organ dysfunction syndrome (n = 23). The established method can be used for determination of p-cresol in the plasma in further clinical research.