ACTH Stimulation Test in Infants: NPO Status Not Required
Infants do not need to be NPO (nil per os) for an ACTH stimulation test. No guideline or consensus statement recommends fasting for ACTH measurement or ACTH stimulation testing, as ACTH and cortisol levels are determined by hypothalamic-pituitary-adrenal axis feedback mechanisms rather than nutritional status 1, 2.
Rationale for No Fasting Requirement
The ACTH stimulation test evaluates adrenal gland responsiveness to exogenous ACTH (cosyntropin), not pituitary ACTH production, and this response is independent of feeding status 3.
Established diagnostic thresholds for the test (peak cortisol ≥18 μg/dL or ≥360 nmol/L) were derived from studies that did not require fasting 1, 4.
Morning timing (08:00-09:00h) is recommended to align with peak endogenous cortisol circadian rhythm, but fasting is not mandated 2.
Practical Testing Considerations in Infants
Standard Protocol
- Perform the test in the morning when possible to standardize results against established reference ranges 2.
- Measure cortisol at baseline, 30 minutes, and 60 minutes after cosyntropin administration 3.
- Use a peak cortisol threshold of ≥18 μg/dL (≥500 nmol/L) or ≥360 nmol/L to define adrenal sufficiency 1, 4.
Age-Specific Nuances
- In term infants <12 months old, the low-dose ACTH stimulation test reliably demonstrates normal adrenal function when peak cortisol ≥20 μg/dL, but has limited value for diagnosing secondary adrenal insufficiency 5.
- Infants with peak cortisol <20 μg/dL may require clinical observation and repeat testing later in infancy for definitive diagnosis 5.
- Both 30- and 60-minute cortisol measurements should be obtained, as 54% of pediatric patients reach peak cortisol at 60 minutes rather than 30 minutes 6.
Important Caveats
- Ensure the infant is not receiving exogenous corticosteroids (dexamethasone, prednisone, hydrocortisone), as these suppress the hypothalamic-pituitary-adrenal axis and invalidate test results 1.
- Maternal antenatal corticosteroid exposure can cause adrenal suppression in very low birth weight infants, affecting cortisol responses in the first two weeks of life 7.
- In critically ill infants with septic shock, absolute adrenal insufficiency is defined by peak ACTH-stimulated cortisol <18 μg/dL 8.
Clinical Application
The infant can feed normally before the test. Focus instead on:
- Scheduling the test in the morning (08:00-09:00h) 2
- Confirming no recent exogenous steroid administration 1
- Obtaining cortisol measurements at both 30 and 60 minutes to avoid missing delayed peak responses 6
- Recognizing that normal weight infants tend to peak at 60 minutes while overweight infants peak at 30 minutes 6