Target analyte quantification by isotope dilution LC-MS/MS directly referring to internal standard concentrations-validation for serum cortisol measurement

Isotope dilution LC-MS/MS methods used in the clinical laboratory
typically involve multi-point external calibration in each
analytical series. Our aim was to test the hypothesis that
determination of target analyte concentrations directly derived
from the relation of the target analyte peak area to the peak area
of a corresponding stable isotope labelled internal standard
compound [direct isotope dilution analysis (DIDA)] may be not
inferior to conventional external calibration with respect to
accuracy and reproducibility.