Recommended ACTH Stimulation Test for Central Adrenal Insufficiency
For a 31-year-old female with central adrenal insufficiency, the low-dose 1 µg ACTH stimulation test is the preferred diagnostic approach, as it provides superior sensitivity compared to the standard 250 µg test for detecting secondary/central adrenal insufficiency 1, 2.
Test Protocol
Low-Dose (1 µg) ACTH Stimulation Test
- Dose: 1 µg synthetic ACTH (cosyntropin/tetracosactide) administered intravenously
- Timing: Measure cortisol at baseline, 30 minutes, and optionally 60 minutes post-injection
- Interpretation: Peak cortisol <500 nmol/L (18 µg/dL) at 30 minutes indicates impaired adrenocortical reserve 1
- Test can be performed at any time of day (not affected by diurnal variation) 2
Why Low-Dose Over Standard-Dose?
The evidence clearly demonstrates that while both tests yield comparable results in primary adrenal insufficiency, the low-dose test provides higher sensitivity for detecting central (secondary) adrenal insufficiency 2. The standard 250 µg dose is supraphysiological and can transiently stimulate the adrenal cortex even in patients with documented central adrenal insufficiency, leading to false-negative results 1.
Meta-analysis data from 30 studies enrolling 1,209 adults showed no statistically significant difference in diagnostic accuracy between high- and low-dose tests for secondary adrenal insufficiency, but the low-dose test demonstrated better sensitivity 3.
Important Caveats for This Patient
Age-Related Considerations
At 31 years old, this patient is of reproductive age. If she is taking estrogen-containing oral contraceptives, cortisol measurements will be falsely elevated due to increased cortisol-binding globulin (CBG) 2. This can lead to false-negative results. Ideally, discontinue oral contraceptives before testing or interpret results with caution.
Assay-Specific Thresholds
The traditional 500 nmol/L (18 µg/dL) threshold was established using older polyclonal antibody assays. Newer monoclonal antibody immunoassays may require lower diagnostic thresholds 4, 5, 6:
- Recent data suggests thresholds as low as 15.6 µg/dL may be more appropriate with newer assays 5
- For the Roche Elecsys Cortisol II assay specifically, a peak cortisol threshold of 127 µg/L has been proposed 4
- Verify which assay your laboratory uses and request assay-specific normative data 2
Alternative Approach: Morning Cortisol First
Before proceeding with dynamic testing, consider measuring morning cortisol (8 AM) at least 24 hours after last glucocorticoid dose (excluding dexamethasone) 7:
- Morning cortisol <150 nmol/L (5 µg/dL): Very likely persistent adrenal insufficiency; dynamic testing unlikely to be useful 7
- Morning cortisol 150-300 nmol/L (5-10 µg/dL): Proceed with low-dose ACTH stimulation test 7
- Morning cortisol >300 nmol/L (10 µg/dL): Suggests HPA axis recovery 7
Recent data using second-generation immunoassays suggests even more refined thresholds:
- Morning cortisol ≤60.9 µg/L predicts central adrenal insufficiency with 96.3% specificity 4
- Morning cortisol >141 µg/L excludes it with 96.2% sensitivity 4
When NOT to Use Low-Dose Test
Do not use the 1 µg test if recent pituitary injury is suspected (within weeks), as the adrenal glands may not have had time to atrophy and could still respond to low-dose stimulation despite true central insufficiency 1.
Practical Limitations
The low-dose test requires dilution of the standard 250 µg vial to 1 µg, which some facilities may not routinely perform. If the 1 µg test is unavailable, the standard 250 µg test is acceptable but recognize its lower sensitivity for central adrenal insufficiency 2. Diluted tetracosactide (in 0.9% saline at 1 µg/mL) is stable at 2-8°C for up to 60 days 2.
Borderline Results
If the low-dose test yields borderline values and the diagnosis remains uncertain, proceed to insulin tolerance test (ITT), which remains the gold standard for diagnosing central adrenal insufficiency 1, 2.