What is the best screening test for Addison's disease?

Best Screening Test for Addison's Disease

The morning 8 AM cortisol level combined with plasma ACTH is the best initial screening test for Addison's disease, with the cosyntropin (Synacthen) stimulation test serving as the definitive confirmatory test when the diagnosis remains uncertain. 1

Diagnostic Algorithm

Initial Screening Approach

When you suspect Addison's disease based on clinical presentation (fatigue, weight loss, hypotension, hyperpigmentation, hyponatremia, hyperkalemia), the diagnostic workup should proceed as follows:

Step 1: Morning (8 AM) Cortisol and ACTH

• Draw blood between 0800-0900 hours for baseline cortisol and ACTH
• In established Addison's disease, you will find:
  • Low cortisol levels
  • Markedly elevated ACTH (>2-fold upper limit of normal)
  • The ACTH/cortisol ratio is elevated in 100% of primary adrenal insufficiency cases 2

Step 2: Cosyntropin Stimulation Test (When Needed)

• Perform when morning cortisol and ACTH are equivocal or when partial adrenal insufficiency is suspected 1
• Administer 0.25 mg cosyntropin (Synacthen) intramuscularly or intravenously
• Measure serum cortisol at 30 and/or 60 minutes
• Normal response: cortisol should exceed 550 nmol/L (approximately 20 μg/dL) 1
• This test has 100% sensitivity for detecting primary adrenal insufficiency 2
• The cosyntropin test is considered the gold standard for confirming the diagnosis 3, 4

Critical Clinical Caveat

If you strongly suspect adrenal crisis, do NOT delay treatment for diagnostic testing. Draw blood for cortisol and ACTH immediately, then start intravenous hydrocortisone 100 mg and 0.9% saline infusion without waiting for results. The diagnosis can be established later, even after treatment has begun. 1

Additional Diagnostic Considerations

Plasma Renin Activity

• Elevated plasma renin activity (PRA) combined with low aldosterone confirms mineralocorticoid deficiency
• The PRA/aldosterone ratio is elevated in 100% of primary adrenal insufficiency cases 2
• This helps distinguish primary from secondary adrenal insufficiency

Etiologic Workup After Diagnosis Confirmation

Once primary adrenal insufficiency is confirmed, determine the cause:

1. First test: 21-hydroxylase antibodies (21OH-Ab)

   • Positive in ~85% of autoimmune Addison's disease in Western populations 1
   • If positive, no further etiologic testing usually needed
   • If negative, proceed to imaging and additional workup

2. If 21OH-Ab negative:

   • CT scan of adrenal glands (look for calcifications suggesting tuberculosis, masses, hemorrhage)
   • In males: measure very long-chain fatty acids (VLCFA) to screen for adrenoleukodystrophy
   • Consider tuberculosis testing in endemic areas 1, 5

Why This Approach is Optimal

The combination of morning cortisol and ACTH provides immediate diagnostic information with high specificity. The 2014 consensus guidelines 1 and 2016 Endocrine Society guidelines 6 consistently emphasize this approach because:

• Morning cortisol alone has limited sensitivity as a screening test 7
• The ACTH level distinguishes primary from secondary causes immediately
• The cosyntropin test provides definitive confirmation when needed, with 100% sensitivity 2
• Early diagnosis improves long-term quality of life, as delays in diagnosis correlate with worse outcomes 6

Common Pitfalls to Avoid

• Don't rely on basal cortisol alone—it's insensitive 7
• Don't perform cosyntropin testing in acute crisis—treat first, diagnose later 1
• Don't assume negative 21OH-Ab excludes autoimmune disease—antibodies can be absent in children, elderly, and may disappear over time 1
• Don't forget to check for associated autoimmune conditions (thyroid disease, type 1 diabetes) in autoimmune Addison's 6, 8