Determination of cortisol production rates with contemporary liquid chromatography–mass spectrometry to measure cortisol-d3 dilution after infusion of deuterated tracer

ObjectivesMeasurement of 24-h cortisol production rate (CPR) using steady-state infusion of deuterated cortisol and analysis of stable-isotope dilution by MS is a valuable tool to examine hypothalamic–pituitary–adrenal axis activity in humans. We have developed and validated an improved method for measuring cortisol dilution with contemporary LC-MS technology.Design and methodsPlasma samples and calibrators were extracted with ethylacetate. LC-MS was performed with a Surveyor HPLC and TSQ Quantum triple-quadrupole mass spectrometer equipped with an atmospheric pressure chemical ionization (APCI) source.ResultsSelectivity was improved over previous methods via elimination of an interferent identified as 20β-dihydrocortisol. The LLOQ for cortisol-d3 was 2.73 nmol/L and LOD 1.37 nmol/L. Plasma calibrators were linear over the concentration range 1.5–10% cortisol-d3, with correlation coefficients > 0.995.ConclusionsThis APCI LC-MS method offers simplified sample work-up and analysis and enables selective detection of the low concentration of cortisol-d3 infused for determination of 24-h CPR.