What is the confirmatory diagnostic test for central (secondary) adrenal insufficiency?

Confirmatory Diagnostic Test for Central Adrenal Insufficiency

The insulin tolerance test (ITT) remains the gold standard confirmatory test for central adrenal insufficiency, though the overnight metyrapone test (OMT) is an excellent alternative when ITT is contraindicated or unavailable 1, 2.

Algorithmic Diagnostic Approach

Step 1: Morning Cortisol (8 AM)

Start with a morning serum cortisol measurement at least 24 hours after the last glucocorticoid dose (excluding dexamethasone):

• <150 nmol/L (<5 μg/dL): Confirms central adrenal insufficiency—dynamic testing unnecessary 1
• >300 nmol/L (>10 μg/dL): Rules out adrenal insufficiency—no further testing needed 1
• 150-300 nmol/L (5-10 μg/dL): Proceed to dynamic testing 1

Recent data using second-generation assays suggest even more refined thresholds: values ≤60.9 μg/L predict CAI with 96.3% specificity, while >141 μg/L excludes it with 96.2% sensitivity 3.

Step 2: Dynamic Testing (When Morning Cortisol Indeterminate)

Primary Options:

1. Insulin Tolerance Test (ITT) - Gold Standard 2, 4

   • Most accurate for central adrenal insufficiency
   • Contraindications: seizure disorders, cardiovascular disease, elderly patients

2. Overnight Metyrapone Test (OMT) - Highly Reliable Alternative 1, 5

   • 11-deoxycortisol >200 nmol/L: 100% sensitivity and 94% specificity for excluding CAI 5
   • Validated against long-term clinical outcomes
   • ACTH measurement adds no diagnostic benefit 5

3. Low-Dose ACTH Stimulation Test (1 μg) - Practical Option 6, 4

   • Peak cortisol <500 nmol/L at 30 minutes indicates impaired reserve 6
   • More sensitive than the standard 250 μg test for central AI
   • Should NOT be used if recent pituitary injury suspected 6

4. Standard ACTH Test (250 μg) - Limited Role 5, 4

   • Inferior sensitivity (75%) compared to OMT 5
   • May be useful only in low-risk patients (CAI-Score = 0) with high negative predictive value 5
   • Supraphysiological dose can transiently stimulate adrenals even in true CAI 6

Key Diagnostic Thresholds

For ACTH stimulation tests:

• Peak cortisol <450 nmol/L (30 min): Suggests CAI 5
• Peak cortisol <500 nmol/L (1 μg test): Impaired reserve 6
• Values between cutoffs require ITT or OMT for definitive diagnosis 4

Critical Pitfalls to Avoid

Assay Variability: Cortisol measurement varies significantly between immunoassays and mass spectrometry 1. Interpret thresholds in context of your laboratory's specific assay.

Timing Issues:

• Cortisol peaks just before waking—falsely low in night shift workers, jet lag, severe insomnia 1
• Must wait ≥24 hours after last glucocorticoid dose 1

Binding Protein Alterations:

• Falsely elevated: Pregnancy, oral estrogens (increased CBG) 1
• Falsely decreased: Cirrhosis, nephrotic syndrome, malnutrition, critical illness (low albumin/CBG) 1

Recent Pituitary Injury: The low-dose ACTH test should NOT be used in acute pituitary injury as the adrenal glands may not yet be atrophied 6.

Adjunctive Markers

DHEA-S may help rule out primary adrenal insufficiency when age- and sex-adjusted 7, though this is more relevant for distinguishing primary from central causes rather than confirming central AI.

Salivary cortisone is emerging as a practical screening tool that avoids CBG variability 7, but is not yet standard for confirmatory diagnosis.

Risk Stratification Approach

For patients with indeterminate morning cortisol, consider the CAI-Score incorporating:

• Morning cortisol level
• Number of pituitary hormone deficits (≥3 deficits: OR 35.38 for CAI) 8
• Pituitary tumor size
• Male sex (OR 1.77) 8

Scores ≥4.5 indicate 70.1% CAI likelihood versus 10.5% at 0 points 8. This helps prioritize who needs immediate dynamic testing versus continued monitoring.

Practical Recommendation

For most clinical scenarios with suspected central adrenal insufficiency and indeterminate morning cortisol, proceed with the overnight metyrapone test given its superior validated performance (100% sensitivity, 94% specificity) compared to ACTH stimulation testing 5. Reserve ITT for cases where OMT is unavailable or results are borderline with high clinical suspicion 4.