Does a serum Adrenocorticotropic Hormone (ACTH) test require a fasting specimen?

ACTH Specimen Collection Requirements

Serum ACTH does NOT require fasting, but MUST be collected in the morning (08:00-09:00h) for accurate diagnostic interpretation. 1, 2

Critical Timing Requirements

• Morning collection (08:00-09:00h) is mandatory because ACTH follows a circadian rhythm with peak levels in the morning and nadir at midnight, making morning values the standardized reference point for all diagnostic thresholds 1, 3
• All established diagnostic cutoffs for ACTH (>5 pg/mL for ACTH-dependent disease, >29 pg/mL for Cushing's disease) are based on morning measurements 1, 4
• The 30-minute cortisol response to ACTH stimulation is constant regardless of time of day, but baseline ACTH interpretation requires morning timing 5, 6

Why Fasting Is NOT Required

• No guideline or consensus statement recommends fasting for ACTH measurement 3, 1
• ACTH levels are determined by the hypothalamic-pituitary-adrenal axis feedback mechanisms, not by nutritional status 3
• The primary concern is circadian timing, not metabolic state 1

Practical Collection Protocol

• Draw blood between 08:00-09:00h after the patient has been awake and upright for at least 30 minutes 1
• Patient does NOT need to be fasting 3, 1
• Ensure patient is not on exogenous steroids, as these suppress ACTH and confound interpretation 2
• For patients with suspected cyclic Cushing's disease, hypercortisolemia must be confirmed immediately prior to ACTH measurement to ensure active disease phase 1

Common Pitfalls to Avoid

• Afternoon ACTH measurements are unreliable because levels are physiologically lower and do not correspond to established diagnostic thresholds 1, 5
• ACTH has a short half-life and levels fluctuate, so single random measurements outside the morning window may be misleading 3
• Medical therapy for Cushing's disease (steroidogenesis inhibitors) must be stopped before ACTH measurement for accurate interpretation 1
• Ensure proper sample handling as ACTH is unstable at room temperature and requires immediate processing 5

Special Considerations for ACTH Stimulation Testing

• For ACTH stimulation tests (used to diagnose adrenal insufficiency, not Cushing's), morning timing is also preferred but the 30-minute cortisol response is constant regardless of basal cortisol level or time of day 5, 6
• The low-dose (1 μg) ACTH stimulation test should measure cortisol at 20 and 30 minutes post-injection for optimal accuracy 7
• Basal cortisol measured between 09:00-13:00h has better diagnostic performance than strict 08:00h morning cortisol for adrenal insufficiency screening 8