Criteria for distinguishing normal and subnormal adrenocortical function using the Synacthen test

The Synacthen test consists of the measurement of plasma corticosteroid concentrations by a fluorimetric method before (basal) and 30 min after a single intramuscular administration of 250 μg of tetracosactrin (Synacthen), a synthetic polypeptide with ACTH activity equivalent to that of natural corticotrophin. In a control group of forty-five subjects the basal plasma corticosteroid level was not lower than 6 μg/100 ml (a lower limit of 5 μg/100 ml is accepted in the afternoon and evening), the increment was not less than 7 μg/100 ml and the plasma corticosteroid concentration after Synacthen was not less than 18 μg/100 ml. From these results the ‘6 (5), 7, 18’ rule was formulated and any patient who failed to satisfy at least two of these criteria was considered to have adrenocortical insufficiency. The results of the Synacthen test were assessed in forty-six patients who had received long-term steroid therapy, and in fifteen patients with suspected or proven pituitary failure or Addison's disease. Using the ‘6 (5), 7, 18’ rule patients who satisfied two or more of the criteria were considered as having normal adrenal function, while patients who failed to satisfy these criteria were diagnosed as having adrenal insufficiency. This separation of patients into those with normal and subnormal adrenal function on the basis of the Synacthen test results was found to be reliable as the same division was obtained when the corticotrophin infusion test was performed on groups of the same patients. The Synacthen test was also carried out in a group of eighty ill patients who did not appear clinically to have endocrine disease and it was considered that the criteria previously determined for distinguishing between a normal and subnormal response to Synacthen were also applicable to this group of patients. It is suggested that using these criteria the Synacthen test is a simple and reliable screening procedure for the detection or exclusion of adrenocortical failure. A total of 215 tests were carried out on 186 patients without any adverse effects.