Specific determination of urinary 6β-hydroxycortisol and cortisol by liquid chromatography–atmospheric pressure chemical ionization mass spectrometry

Abstract The urinary 6β-hydroxycortisol to cortisol ratio is believed to be a noninvasive index of cytochrome P450 3A activity. For precise assessment of the ratio in human urine, we have developed a reversed-phase high-performance liquid chromatography–atmospheric pressure chemical ionization mass spectrometry method. The selective method was accurate and reproducible with intra- and inter-day precision of variation coefficients of less than 8%. The 6β-hydroxycortisol to cortisol ratio ranged from 3.0 to 12.4 in healthy Japanese 24-h urine. With the recent popularization of LC–MS, our LC–MS method will be advantageous to detect human in vivo CYP3A activity for clinical investigation and routine measurement in various laboratories.