Midazolam and 1-hydroxymidazolam are important compounds in drug metabolism for phenotyping and/or drug interaction studies. Like midazolam, more than 50% of all drugs are metabolized by the P-450 CYP3A4 enzyme. Thus measurement of the ratios of midazolam and 1-hydroxymidazolam in the plasma of patients dosed with midazolam can determine their phenotype and if co-administered with another pharmaceutical can determine CYP3A4 drug-drug interactions.