Development of an assay for urinary free cortisol determination on the Technicon Immuno 1 system.
OBJECTIVE
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.


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.
Letellier
M
M
Department of Clinical Biochemistry, Centre Universitaire de Santé de l'Estrie, Sherbrooke (Québec), Canada.
Lévesque
A
A
Daigle
F
F
D003429:Cross Reactions; D005069:Evaluation Studies as Topic; D005260:Female; D006801:Humans; D006854:Hydrocortisone / Q000378:metabolism / Q000652:urine*; D016014:Linear Models; D008297:Male; D011863:Radioimmunoassay; D012016:Reference Values; D012680:Sensitivity and Specificity; D016482:Urinalysis / Q000295:instrumentation* / Q000379:methods*
D006854:Hydrocortisone