ACTH Level Collection During ACTH Stimulation Testing
ACTH levels are not routinely collected during an ACTH stimulation test—the test measures only cortisol response at baseline, 30 minutes, and 60 minutes after cosyntropin administration. 1, 2
Purpose and Methodology of the ACTH Stimulation Test
The ACTH stimulation test (cosyntropin test) is designed to evaluate adrenal gland function, not pituitary ACTH production. 1, 3
- The standard high-dose test uses 250 μg of synthetic ACTH (cosyntropin) administered intravenously or intramuscularly 2
- Cortisol levels are measured at baseline (time 0), 30 minutes, and 60 minutes after administration 1, 4
- A peak cortisol level below 18 μg/dL indicates adrenal insufficiency 1, 4
When ACTH Levels ARE Measured (Separately)
ACTH levels are measured separately from the stimulation test to differentiate primary from secondary adrenal insufficiency:
- Primary adrenal insufficiency: Low cortisol with elevated ACTH (the pituitary is trying to stimulate failed adrenal glands) 5, 1
- Secondary adrenal insufficiency: Low cortisol with low or normal ACTH (pituitary/hypothalamic failure) 5, 1
These baseline ACTH measurements are typically drawn as morning samples on a different occasion, not during the stimulation test itself. 5, 1
Clinical Rationale
The stimulation test bypasses the pituitary entirely by giving exogenous ACTH directly. 2
- The test evaluates whether the adrenal glands can respond to maximal ACTH stimulation 2
- Measuring endogenous ACTH during the test would be meaningless because you've just administered a supraphysiologic dose of synthetic ACTH 2, 6
- The 250 μg dose produces maximal adrenal stimulation equivalent to 25 units of natural ACTH 2
Common Pitfall to Avoid
Do not confuse the diagnostic workup for adrenal insufficiency (which includes baseline ACTH measurement) with the ACTH stimulation test protocol itself. The baseline ACTH helps determine the type of adrenal insufficiency, while the stimulation test determines if adrenal insufficiency exists. 5, 1
Test Interpretation in Context
- In critically ill patients, a delta cortisol <9 μg/dL after cosyntropin defines Critical Illness-Related Corticosteroid Insufficiency (CIRCI) 3
- Some patients peak at 60 minutes rather than 30 minutes, so both time points should be measured to avoid overdiagnosing adrenal insufficiency 4
- The high-dose test is preferred over low-dose (1 μg) testing due to standardized protocols and established reference ranges, despite the low-dose test's theoretical advantages 1, 6