30 minute ACTH test. Reproducibility and interpretation

A 30-minute ACTH stimulation test was performed four times with one week intervals in eighteen normal persons. The coefficient of variation of the average intra-individual peak value in plasma cortisol was significantly (P<0.001) lower than the corresponding coefficient of the basal value and the increase in plasma cortisol after ACTH (peak value 6.2%, basal value 17.8%, and increase 20.1%). Four subjects had an insufficient increase in plasma cortisol in one or more tests, while the peak values were normal in all tests. This observation is explained by an inverse correlation between the basal value and the increase in plasma cortisol, implying a potential subnormal increase when the basal value happens to be high. In the interpretation of the test, it seems reasonable to determine the peak value only, as this parameter has demonstrated the greatest reproducibility.