Clinical inquiries. What's the most practical way to rule out adrenal insufficiency?

A morning serum cortisol level >13 mcg/dL reliably rules out adrenal insufficiency, and the test is easy and safe to perform. Because of low specificity, patients with a level of < or =13 mcg/dL need further evaluation with the cosyntropin stimulation test (CST). The 250 mcg CST requires intravenous (IV) or intramuscular (IM) administration of cosyntropin and multiple blood draws; a normal response reliably rules out primary adrenal insufficiency and moderately decreases the likelihood of secondary adrenal insufficiency. The 1 mcg CST has better diagnostic discrimination, but requires an extra step to dilute the cosyntropin.