Evaluation of a radioimmunoassay of urinary cortisol without extraction.
We evaluated the Kallestad "Quanticoat" cortisol RIA for direct (no extraction) measurement of urinary free cortisol, which requires no solvent extraction. An analytical-recovery study showed a linear regression of y (measured) = 0.65x (added) + 37.5 micrograms/L (Sy.x = 21.4 micrograms/L, r = 0.978, n = 48). Recovery appeared to vary with the urine used and with the concentrations of cortisol added. Within- and between-run CVs were less than or equal to 4.1% and less than or equal to 3.8%, respectively. Cross reactivities were low, except for prednisolone (20.5%). This no-extraction method gave higher values for urinary free cortisol than did either an RIA method involving extraction or an HPLC method. A comparison study with the HPLC (x) and with the method involving extraction (x') gave the following Deming-debiased regression equations: y = 1.60x + 68.8 (Sy.x = 34.4, n = 29) and y = 1.33x' + 0.69 (Sy.x = 40.3, n = 66), respectively. We conclude that the no-extraction method may give misleading results for patients' diagnosis or management if this cross reactivity is not taken into account.
Huang
C M
CM
Clinical Pathology Department, Warren Grant Magnuson Clinical Center, Bethesda, MD 20892.
Zweig
M
M
D002851:Chromatography, High Pressure Liquid; D003350:Cortodoxone / Q000652:urine; D006801:Humans; D006854:Hydrocortisone / Q000652:urine*; D011239:Prednisolone / Q000652:urine; D011786:Quality Control; D011863:Radioimmunoassay / Q000592:standards*; D011933:Reagent Kits, Diagnostic / Q000592:standards*; D012044:Regression Analysis
D011933:Reagent Kits, Diagnostic; D011239:Prednisolone; D003350:Cortodoxone; D006854:Hydrocortisone