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Clinical Chemistry 35: 125-126, 1989;
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Clinical Chemistry, Vol 35, 125-126, Copyright © 1989 by American Association for Clinical Chemistry

Evaluation of a radioimmunoassay of urinary cortisol without extraction

CM Huang and M Zweig
Clinical Pathology Department, Warren Grant Magnuson Clinical Center, Bethesda, MD 20892.

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.


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Copyright © 1989 by the American Association for Clinical Chemistry.