ACTH Estimation: Recommended Method
The recommended method for estimating ACTH levels is a morning (08:00-09:00h) plasma ACTH measurement obtained via standard venipuncture, with interpretation based on the clinical context of cortisol levels and suspected diagnosis. 1
Optimal Timing and Collection Method
Morning collection between 08:00-09:00h is the standard reference point for ACTH measurement, as cortisol and ACTH follow a diurnal rhythm with highest levels in the morning, allowing for standardization across patients and comparison with established reference ranges. 1
Plasma ACTH should be collected in EDTA tubes, placed immediately on ice, and processed rapidly, as ACTH is highly unstable at room temperature. 2
For patients with cyclical Cushing's disease, hypercortisolemia must be confirmed immediately prior to ACTH measurement to ensure the patient is in an active disease phase. 1
Interpretation Framework
For Primary Adrenal Insufficiency:
- Paired measurement of serum cortisol and plasma ACTH is the diagnostic test of choice. 2
- Serum cortisol <250 nmol/L with increased ACTH in the presence of acute illness is diagnostic of primary adrenal insufficiency. 2
- Serum cortisol <400 nmol/L with increased ACTH in acute illness raises strong suspicion of primary adrenal insufficiency. 2
For Cushing's Syndrome Classification:
- ACTH levels definitively determine whether Cushing's syndrome is ACTH-dependent or ACTH-independent. 1
- Any ACTH level >5 ng/L (>1.1 pmol/L) is detectable and suggests ACTH-dependent Cushing's syndrome with high certainty. 1
- ACTH >29 ng/L has 70% sensitivity and 100% specificity for diagnosing Cushing's disease. 1
- Low or undetectable ACTH levels indicate ACTH-independent Cushing's syndrome from an adrenal source. 1
Alternative Testing Methods: Not Recommended for Routine Use
ACTH Stimulation Testing vs. Direct ACTH Measurement:
- The 250-μg ACTH stimulation test is recommended for diagnosing critical illness-related corticosteroid insufficiency (CIRCI), but this tests adrenal reserve rather than estimating baseline ACTH levels. 2
- Direct plasma corticotropin (ACTH) level measurement is not recommended for routine diagnosis of CIRCI due to complexity of measurement and inability to obtain results with sufficiently short turnaround time to impact acute management. 2
Salivary Cortisol:
- Salivary cortisol measurement is not recommended for routine use, as evidence demonstrating benefit over serum cortisol is extremely limited, and practical concerns include lack of routine availability of enzyme immunoassay and high rates of insufficient samples or contamination. 2
Critical Pitfalls to Avoid
Do not rely on ACTH levels alone without paired cortisol measurement, as interpretation requires understanding the cortisol-ACTH relationship. 2
Avoid afternoon or evening ACTH collection for initial assessment, as this eliminates the ability to compare with established morning reference ranges. 1
Stop medical therapy for Cushing's disease (steroidogenesis inhibitors) before diagnostic procedures to enable accurate interpretation of results. 1
Exogenous steroid use (oral prednisolone, dexamethasone, or inhaled fluticasone) may confound interpretation of low serum cortisol levels. 2
Be aware that ACTH precursor peptides are proportionately more elevated in ectopic ACTH syndrome associated with small cell lung cancer compared to pituitary-dependent Cushing's syndrome, though specialized IRMA assays are required for this distinction. 3