Technical details influence the diagnostic accuracy of the 1 μg ACTH stimulation test

Objective: To examine the factors causing inadequate cortisol responses to the 1 mu g ACTH stimulation test. Design: Random test assignment (by age and gender) at 0800 or 1600 h. Methods: We recruited 20 healthy adults to each of the three age groups (< 40 years, 40-55 years, and > 55 years; half females in each group). ACTH stimulation tests were performed in an outpatient clinic at the NIH Clinical Research Center. Plasma cortisol was measured just before, and 30 and 60 min after the administration of 1 mu g ACTH (1-24). The ACTH concentration in diluted and administered solutions was measured. Results: Twenty-five volunteers (19 at 1600 h) had a subnormal cortisol response (peak cortisol 10.4-17.5 mu g/dl), using a criterion < 18 mu g/dl (497 nmol/l), for a specificity of 58% (confidence interval (CI) 45-71%). Afternoon testing had a significant impact on failure rates (odds ratio 6.98, CI 2.17-22.43), while gender and age did not. The stock solution contained 1 mg ACTH, but after administration through tubing it contained only 0.5-0.8 mu g. Conclusions: The high rate of abnormal results, especially in the afternoon, and loss of ACTH through tubing suggest that morning testing and minimal tubing should be adopted to avoid an inappropriate diagnosis of adrenal insufficiency. Earlier time points and standardized protocols would facilitate comparison of studies.