Is starting an Adrenocorticotropic hormone (ACTH) stim test in a patient with suspected adrenal insufficiency at a later time too late?

Timing of ACTH Stimulation Test

The ACTH stimulation test can be performed at any time of day—there is no "too late" in terms of timing, though morning testing is preferred for convenience rather than necessity. 1

Test Timing Considerations

• The short Synacthen test should preferably be performed in the morning, although it is not strictly necessary 1
• Unlike baseline cortisol measurements (which must be drawn at 8 AM to capture the physiologic peak), the ACTH stimulation test evaluates adrenal reserve capacity, which remains constant throughout the day 1
• The test measures the adrenal glands' ability to respond to exogenous ACTH stimulation—this functional capacity does not vary significantly by time of day 1

Standard Test Protocol

• Administer 0.25 mg (250 mcg) cosyntropin intramuscularly or intravenously 1
• Obtain baseline serum cortisol and ACTH before administration 1
• Measure serum cortisol at 30 and/or 60 minutes post-administration 1
• A peak cortisol value above 500-550 nmol/L (18-20 μg/dL) is considered normal, while <500 nmol/L is diagnostic of adrenal insufficiency 1, 2

Critical Timing Exception: Suspected Adrenal Crisis

• If the patient is clinically unstable with suspected adrenal crisis, treatment should NEVER be delayed for diagnostic procedures 1
• Immediately administer 100 mg IV hydrocortisone plus 0.9% saline infusion at 1 L/hour if there is unexplained hypotension, collapse, or gastrointestinal symptoms 1
• Draw blood for cortisol and ACTH before treatment if possible, but do not delay therapy 1
• If you need to treat but still want diagnostic testing later, use dexamethasone 4 mg IV instead of hydrocortisone, as dexamethasone does not interfere with cortisol assays 1

Practical Considerations

• Morning testing is recommended primarily for logistical convenience and to allow simultaneous baseline cortisol/ACTH measurement if not already obtained 1
• The high-dose (250 mcg) test is preferred over low-dose (1 mcg) due to easier practical administration, comparable diagnostic accuracy, and FDA approval 1
• Ensure the patient has not taken exogenous steroids (prednisolone, inhaled fluticasone) that could suppress the HPA axis and confound results 1