Figure 1.

Method comparison between indirect quantification of unbound p-cresyl sulfate concentrations after hydrolysis to p-cresol by gas chromatography-mass spectrometry and direct quantification of unbound p-cresyl sulfate concentrations by HPLC. Deming fit demonstrated excellent agreement. As expected, we observed a nonsignificant (P = 0.1) small proportional bias (y = 0.00 + 0.91 · x). Indeed, besides p-cresyl sulfate, small amounts of p-cresyl glucuronide and unconjugated p-cresol are quantified by the indirect GC-MS method.