What are the typical Canadian laboratory units used for diagnosing Addison's disease?

Canadian Laboratory Units for Diagnosing Addison's Disease

In Canada, the key diagnostic laboratory values for Addison's disease use the following units: serum cortisol in nmol/L, ACTH in pmol/L (or pg/mL), sodium and potassium in mmol/L, with a cosyntropin stimulation test requiring peak cortisol >550 nmol/L to exclude adrenal insufficiency. 1

Initial Diagnostic Laboratory Panel

• Morning serum cortisol should be measured in nmol/L, with values <250 nmol/L (<9 μg/dL) plus elevated ACTH being diagnostic of primary adrenal insufficiency in the setting of acute illness 1, 2
• Plasma ACTH is measured in pmol/L or pg/mL, with markedly elevated levels (>300 pg/mL when cortisol <140 nmol/L) indicating primary adrenal insufficiency 1, 3
• Serum sodium is reported in mmol/L, with hyponatremia present in approximately 90% of newly diagnosed cases 1
• Serum potassium is reported in mmol/L, with hyperkalemia occurring in about 50% of patients at diagnosis 1

Cosyntropin Stimulation Test Parameters

• The standard test involves administering 0.25 mg (250 mcg) cosyntropin intramuscularly or intravenously 1, 2
• Serum cortisol is measured at baseline, 30 minutes, and/or 60 minutes post-administration 1
• A peak cortisol >550 nmol/L (>18-20 μg/dL) at either 30 or 60 minutes is considered normal and excludes adrenal insufficiency 1, 2
• A peak cortisol <500-550 nmol/L (<18 μg/dL) is diagnostic of adrenal insufficiency 1, 2

Additional Laboratory Findings

• Aldosterone is measured in ng/dL or pmol/L, with low levels indicating mineralocorticoid deficiency 4
• Plasma renin activity is measured in mg/dL/hour or ng/mL/hour, with elevated levels (>6.0 mg/dL/hour) supporting primary adrenal insufficiency 4
• Mild to moderate hypercalcemia, anemia, mild eosinophilia, and lymphocytosis are common findings 1
• Elevated liver transaminases may be present 1

Critical Diagnostic Caveats

• Normal cortisol does not exclude early Addison's disease - approximately 10% of patients with confirmed primary adrenal insufficiency present with normal basal cortisol concentrations (>350 nmol/L) but have markedly elevated ACTH 4, 3
• The combination of normal-to-high cortisol with clearly elevated ACTH (>300 pg/mL) is indicative of evolving primary adrenal insufficiency when clinical history is suggestive 3
• Basal cortisol between 250-400 nmol/L with elevated ACTH in acute illness generates strong suspicion and requires cosyntropin stimulation testing 2

Etiologic Testing

• 21-hydroxylase autoantibodies should be the first test to establish autoimmune etiology, accounting for approximately 85% of cases 1, 5
• If autoantibodies are negative, CT imaging of the adrenals and measurement of very long-chain fatty acids should be performed 1

Important Clinical Pitfall

• Treatment should never be delayed for diagnostic testing if adrenal crisis is suspected - administer 100 mg IV hydrocortisone immediately and draw blood for cortisol and ACTH before treatment if possible 1, 2, 5