Reproducibility of the cortisol response to stimulation with the low dose (1 μg) of ACTH

OBJECTIVE Previous studies have shown that the rapid ACTH stimulation test using a low dose of mu g is more sensitive than that using 250 mu g ACTH for detecting subtle cases of adrenal insufficiency. However, there are controversies for the reproducibility of the 1 mu g-test. To evaluate the reproducibility of the 1 mu g-test, we assessed both day-to-day and diurnal variations of cortisol responses to 1 mu g ACTH injection, In addition, optimum sampling time for the 1 mu g-test was also determined, SUBJECTS AND DESIGN Eight healthy Volunteers and five patients with secondary adrenal insufficiency were recruited, Healthy subjects were given 1 mu g ACTH 3 times in the morning (0800 h) and 2 times in the afternoon (1600 h), Patients with adrenal insufficiency had P-tests in the morning and 2 in the afternoon. Serum cortisol levels were measured every 10 minutes for 1 h after the injection, RESULTS In healthy subjects, basal and peak serum cortisol levels were significantly higher in the morning (P<0.05), whereas maximum cortisol increments were higher in the afternoon (P<0.001). In patients with adrenal insufficiency, basal and peak serum cortisol levels in the morning were not different from corresponding values in the afternoon, Intra-individual coefficient of variation (CV) of peak serum cortisol response to 1 pg ACTH ranged from 3.0 to 16.4% in healthy subjects and 10.0-34.4% in patients, Also, there was a significant correlation between peak morning or afternoon cortisol levels after 1 mu g ACTH injection given in different days in both healthy subjects and patients. Twenty-six of the 40 studies in healthy subjects showed peak response at 20 minutes, while nine showed it at 30 minutes Using the data acquired at 20 and 30 minutes, all 40 studies in healthy subjects showed normal results while none of 20 studies in patients was normal, CONCLUSIONS We conclude that the cortisol response to 1 mu g ACTH stimulation was reproducible in both healthy subjects and patients with secondary adrenal insufficiency, In order to assess adrenal function more accurately with the 1 mu g ACTH stimulation test, serum cortisol should be measured before and 20 and 30 minutes after ACTH injection.