How to Perform the Cosyntropin Test for Central Adrenal Insufficiency
The cosyntropin stimulation test has limited utility for diagnosing central (secondary) adrenal insufficiency and should not be routinely performed; instead, prioritize morning cortisol measurement as the initial screening test, reserving dynamic testing only for indeterminate cases. 1
Critical Limitation for Central AI
The cosyntropin test is not the gold standard for secondary adrenal insufficiency 2. Unlike primary adrenal insufficiency where it performs well, the standard 250 μg dose can transiently stimulate adrenals even in patients with documented central AI, leading to false negatives 3. The insulin tolerance test remains the true gold standard for central AI, though it's rarely performed due to safety concerns.
Initial Screening Approach
Start with morning (8 AM) serum cortisol:
- >10-13 μg/dL (>300 nmol/L): Rules out AI, no further testing needed 1, 4
- <5 μg/dL (<140 nmol/L): Confirms AI, proceed with treatment 4
- 5-13 μg/dL: Indeterminate zone requiring dynamic testing 4
This approach obviates the need for cosyntropin testing in most patients 1.
If Cosyntropin Testing is Necessary
Test Protocol
Standard Dose (250 μg) - FDA Approved 5:
- Dose: 0.25 mg (250 μg) IV or IM bolus
- Timing: Obtain cortisol at baseline, exactly 30 minutes, and 60 minutes post-injection
- No time constraints: Unlike morning cortisol, can be performed any time of day 2
Low Dose (1 μg) - More Sensitive for Central AI 2, 6, 3:
- Evidence suggests the 1 μg test has higher sensitivity for secondary AI than the 250 μg test 2, 6, 3
- Requires dilution of standard vial (stable at 2-8°C for 60 days in 0.9% saline) 2
- Use only when cosyntropin is in short supply or for suspected central AI 2
- Measure cortisol at 30 minutes
Pre-Test Medication Management 5
Stop on day of testing:
- Glucocorticoids (short-acting)
- Spironolactone
Stop 4-6 weeks before testing:
- Estrogen-containing medications (elevate cortisol-binding globulin, falsely raising total cortisol) 5
Stop longer period before testing:
- Long-acting glucocorticoids (dexamethasone)
Interpretation Thresholds
For 250 μg dose:
- Peak cortisol ≥18 μg/dL (500 nmol/L): Traditional threshold, but assay-dependent 2
- Newer monoclonal antibody assays may require lower thresholds (14.6-17.0 μg/dL) 7, 8, 9
- Peak typically occurs at 30 or 60 minutes; measure both to avoid missing late peakers 10
For 1 μg dose:
- Peak cortisol ≥18-20 μg/dL at 30 minutes 6, 3
- Some evidence supports ≥16.1 μg/dL with appropriate validation 11
Critical Pitfalls
Assay variability is substantial:
- Different immunoassays show baseline cortisol cutoffs ranging from 336-506 nmol/L (12.2-18.3 μg/dL) for the same clinical scenario 1
- Know your laboratory's specific cutoffs and assay method 1, 2
- Mass spectrometry is most accurate but rarely used 1
Conditions affecting interpretation:
- Elevated CBG (pregnancy, oral estrogens): Falsely high total cortisol 1, 5
- Low CBG/albumin (cirrhosis, nephrotic syndrome, critical illness): Falsely low total cortisol 1
- Disrupted circadian rhythm (night shift, jet lag): Affects morning cortisol only 1
Timing considerations:
- Test is unreliable in acute/recent pituitary injury (<4-6 weeks) as adrenals haven't atrophied yet 3
- In chronic secondary AI, adrenal atrophy develops over weeks, making the test more reliable
When to Use Alternative Tests
Consider insulin tolerance test or overnight metyrapone test when:
- Cosyntropin test is indeterminate (cortisol 16-20 μg/dL range) AND strong clinical suspicion persists 4
- Recent pituitary surgery/injury (<6 weeks) 3
- Multiple pituitary hormone deficiencies present 4
Measure baseline ACTH simultaneously:
- In central AI, ACTH should be low or inappropriately normal (not elevated) 2
- Helps differentiate primary (high ACTH) from secondary (low/normal ACTH) AI