Establishing a reference range for urine cortisol:creatinine ratio
Rebecca Reynolds1, Clement Ho2, Louise Sawyers2, Kathleen Gough2 & Catherine Shearing2
Background: Measurement of urinary cortisol is commonly used in the investigation of suspected Cushings Syndrome and is most often performed using immunoassay. This approach exhibits significant assay variability necessitating the use of method specific reference ranges. A reference range for cortisol nmol/L: creatinine mmol/L ratio is reported based on the results from 104 early morning urine samples collected from healthy individuals and analysed using the AutoDelfia Automated Immunoassay Analyser.
Methods: Subject details including age, sex, height, weight and current medication were recorded. Prior to analysis samples were stored at 20C. Measurement involved a solvent extraction step, evaporation to dryness and reconstitution in a human serum based matrix prior to assay using a competitive time-resolved fluoroimmunoassay (Perkin Elmer). Creatinine was analysed using dry slide chemistry on the Fusion analyser (Ortho Diagnostics). Cortisol:creatinine values were log-transformed for analysis. Samples from 8 outliers were excluded from analysis and are under further investigation.
Results: The subjects (male, n=54; females, n=50) were of mean age 40 (S.D. 12.8) yrs and BMI 24.2 (S.D. 4.16) kg/m2. None had taken glucocorticoid therapy by any route for 3 months prior to sample collection. 8 women were taking the oral contraceptive pill, and 1 woman was taking hormone replacement therapy. The between assay coefficients of variation of the cortisol assay were 8.5, 7.0 and 11.3% at mean (n) levels of 87 (11), 227 (11) and 442 (10) nmol/l respectively. The mean urine cortisol:creatinine ratio (± 1.96 sds) was 11.55 (4.0, 33.0). The ratio did not vary according to BMI, age or gender.
Conclusions: A clearly defined method related reference range for the cortisol: creatinine ratio in early morning urine has been described. Early morning urine sampling is a simple procedure compared to a 24 hour urine sample collection. The use of this measurement in the investigation and diagnosis of Cushings syndrome remains to be evaluated.