Determination of serum cortisol using isotope dilution-liquid chromatography-mass spectrometry as a candidate reference method.
We propose isotope-dilution mass spectrometry as a candidate reference method for determination of serum cortisol. The method uses liquid chromatography-mass spectrometry (LC-MS), interfaced with electrospray ionization, and selective monitoring of the [M + H]+ ions of cortisol and isotopically labeled cortisol. The isotope-dilution-liquid chromatography-mass spectrometry (ID-LC-MS) method simplifies sample-preparation, because samples are processed by simple solvent extraction without further clean-up and derivatization. We studied the time required for complete equilibration of endogenous cortisol and labeled cortisol spiked into serum and found it to be less than 1 h. The repeatability and the reproducibility of the method were evaluated and found to be 0.55% of the measurement value. CRM 192 and 193 from the Bureau Communautaire de Reference were analyzed for verification of the method. The results obtained from the ID-LC-MS method agreed with the certified values. The relative uncertainty of measurement results for samples in the range of a few tens of micrograms per kilogram to several hundred micrograms per kilogram was evaluated and found to be 0.56%. Immunoassay carried out by three independent clinical laboratories produced results more than 15% higher than this ID-LC-MS method, suggesting the presence of bias in the immunoassay methods.
Department of Chemistry, KyungNam University, Masan, Kyungnam 631-701, Korea.
Pyong Gil
PG
Jung
Byungjoo
B
Kim
Sang-Ryoul
SR
Park
Hun-Young
HY
So
Lian Hua
LH
Shi
Yongseong
Y
Kim
D008401:Gas Chromatography-Mass Spectrometry / Q000379:methods*; D006854:Hydrocortisone / Q000097:blood*; D007118:Immunoassay / Q000379:methods; D007554:Isotopes / Q000737:chemistry*; D012016:Reference Values; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013997:Time Factors
D007554:Isotopes; D006854:Hydrocortisone