Abstract
Urinary free cortisol (UFC) is used to assess disease activity in hypercortisolemic
patients. However, reference ranges are often lacking, especially with respect to
potential confounding variables. This study analyzed upper limits of normal (ULN,
mean + 2 SD) for 2 newer immunoassays, using gas chromatography-mass spectrometry
(GC-MS) as reference method. Each 10 healthy subjects were grouped by age (18–29;
30–49; ≥ 50 years), BMI (< 25; ≥ 25 kg/m2), and sex, resulting in a total of 120 controls (60 males; age: 39.3±1.3 years; BMI:
25.9±0.4 kg/m2). ULN were calculated for a radioimmunoassay (RIA, Immunotech) and an electrochemiluminescence
immunoassay (ECLIA, Roche) and applied to 12 hypercortisolemic patients (4 males;
age: 53.1±3.1 years; BMI: 29.1±1.8 kg/m2). To determine degradation, samples were stored at 4°C (without light) or 22°C (with
and without light) for 0, 24, and 72 h. Cortisol concentrations were significantly
correlated: r=0.88 for RIA vs. ECLIA, r=0.75 for RIA vs. GC-MS, and r=0.77 for ECLIA
vs. GC-MS (always p<0.0001). For each procedure, multiple stepwise regression analysis
identified sex as the only significant predictor, resulting in sex-dependent ULN (males
vs. females): 294 vs. 208 nmol/24 h (RIA), and 379 vs. 277 nmol/24 h (ECLIA). These
ULN classified samples from patients as hypercortisolemic in 100% (RIA) and 95% (ECLIA).
Different storage conditions over 72 h did not alter UFC levels significantly. Results
of the 3 procedures were well correlated, and the use of assay- and sex-specific ULN
allowed excellent identification of hypercortisolic states. UFC is stable over 72 h
irrespective of the storage conditions applied.
Key words
age - body mass index (BMI) - cortisol - gender - reference range - urinary free cortisol
(UFC) - storage
