On the stability in vitro of bioactive human adrenocorticotrophin in blood and plasma.

The disappearance in vitro of ACTH from whole human blood and plasma held at 22 degrees C has been monitored using a dispersed adrenal cell bioassay and an unextracted radioimmunoassay. In three normal subjects after a metyrapone test and two patients with Addison's disease, endogenous bioactive ACTH levels were unchanged for at least 1 h in blood and 2 h in plasma. Moreover greater than 50% of the bioactive plasma ACTH was still present in the plasma samples from the patients with Addison's disease after 24 h incubation at ambient temperatures. Human pituitary ACTH (1-39), spiked into plasma from dexamethasone suppressed subjects to give a concentration of 250 ng/l, was stable by bioassay for at least 2 h. No loss of biological activity was observed on subjecting plasma from a patient with Addison's disease or spiked plasma to two cycles of thawing at 37 degrees C and freezing at -70 degrees C or thawing at 20 degrees C and freezing at -20 degrees C. Some loss of bioactivity (20%) occurred on subjecting the patient's, but not ACTH-spiked plasma to four cycles of thawing at 20 degrees C/freezing at -20 degrees C. We conclude that bioactive ACTH (endogenous or exogenous) may be more stable in vitro in human blood and plasma than has been previously thought. If our studies can be confirmed in a larger series then it may be that conditions for handling blood specimens for ACTH assays could be reappraised.