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Development of an assay for urinary free cortisol determination on the Technicon Immuno 1 system
M Letellier1, A Lévesque, F Daigle
1Department of Clinical Biochemistry, Centre Universitaire de Santé de l'Estrie, Sherbrooke (Québec), Canada.
Objectives:
To develop and evaluate a method using an ethyl acetate extraction procedure for the determination of urinary free cortisol on the Technicon Immuno 1 system from Bayer Corporation.
Design And Methods:
We tested the assay precision, linearity, and correlation with the Urinary Kallestad Quanticoat Cortisol radioimmunoassay. We also studied the efficiency of the extraction procedure, performed a cross-reactivity study with different cortisol metabolites, and determined the reference values.
Results:
The assay shows within-run CVs varying from 1.6 to 5.3% and between-day CVs from 2.7 to 6.1% for urinary free cortisol concentrations from 58 to 1097 nmol/L. The assay demonstrates an excellent linearity and a very good correlation with the Kallestad Quanticoat Cortisol assay (slope = 0.94, y-intercept = 29 nmol/L, Sy/x = 54 nmol/L, r = 0.996). The reference values were estimated at 42-281 nmol/d. The extraction procedure shows an average recovery of 99.0% and minimal interference with the cortisol metabolites tested with the exception of cortisone.
Conclusions:
The evaluation shows that the developed assay has the analytical characteristics required for its utilization in a clinical laboratory.