What defines a positive cortisol stimulation test in a patient suspected of having adrenal insufficiency?

Defining a Positive Cortisol Stimulation Test

A positive cortisol stimulation test—meaning one that confirms adrenal insufficiency—is defined by a peak cortisol level below 18 μg/dL (500-550 nmol/L) at either 30 or 60 minutes after administration of 0.25 mg cosyntropin. 1, 2, 3, 4

Standard Test Protocol

The cosyntropin (ACTH) stimulation test follows this protocol:

• Administer 0.25 mg (250 mcg) cosyntropin intravenously or intramuscularly 1, 2, 4
• Obtain baseline serum cortisol immediately before administration 2, 4
• Measure serum cortisol at exactly 30 minutes and 60 minutes after cosyntropin administration 1, 2, 4
• The test is preferably performed in the morning, though not strictly required 2

Diagnostic Thresholds

Peak cortisol <18 μg/dL (500-550 nmol/L) at 30 or 60 minutes is diagnostic of adrenal insufficiency. 1, 2, 3, 4 This is the consensus threshold across major guidelines, though the exact cutoff varies slightly:

• The FDA label for cosyntropin states that stimulated cortisol levels <18 mcg/dL at 30 or 60 minutes are suggestive of adrenocortical insufficiency 4
• The Endocrine Society recommends <500-550 nmol/L (<18-20 μg/dL) as diagnostic 1, 2
• Peak cortisol >550 nmol/L (>18-20 μg/dL) is considered normal and excludes adrenal insufficiency 1, 2

Assay-Specific Considerations

Critical caveat: These thresholds were established for polyclonal antibody-based immunoassays. Newer monoclonal antibody-based assays may require lower cutoffs 5:

• For the Abbott Architect assay, the optimized threshold is 14.6 μg/dL at 60 minutes (sensitivity 92%, specificity 96%) 5
• For the Roche Elecsys II assay, lower thresholds than the historical 18 μg/dL have been proposed 5
• Using the traditional 18 μg/dL cutoff with these newer assays may result in false-positive diagnoses and overtreatment 5

Special Populations and Contexts

Critically Ill Patients

In critically ill patients with suspected critical illness-related corticosteroid insufficiency (CIRCI), alternative criteria apply:

• A random plasma cortisol <10 μg/dL may be diagnostic without requiring stimulation testing 1, 2
• A delta cortisol <9 μg/dL (change from baseline at 60 minutes after cosyntropin) may indicate CIRCI 1, 3
• The high-dose (250 mcg) test is still recommended over the low-dose (1 mcg) test in this population 1, 3

Pediatric Dosing

For pediatric patients, the dosing differs 4:

• Birth to <2 years: 0.125 mg (0.5 mL of reconstituted solution) 4
• 2 to 17 years: 0.25 mg (1 mL of reconstituted solution) 4

Critical Pitfalls to Avoid

Medication Interference

Stop these medications before testing to avoid false results:

• Glucocorticoids and spironolactone should be stopped on the day of testing; long-acting glucocorticoids may require longer washout 1, 4
• Estrogen-containing drugs must be stopped 4-6 weeks before testing because they elevate cortisol-binding globulin, artificially raising total cortisol levels 2, 4
• Exogenous therapeutic steroids (dexamethasone, prednisone, prednisolone) suppress the HPA axis and interfere with testing 1, 2

Conditions Affecting Cortisol-Binding Globulin

Any condition that alters cortisol-binding globulin (CBG) levels will affect total cortisol measurements 4:

• Low CBG (cirrhosis, nephrotic syndrome) falsely lowers total cortisol 4
• High CBG (pregnancy, estrogen therapy) falsely elevates total cortisol 4
• Consider measuring free cortisol or CBG levels when these conditions are present 6

When NOT to Perform the Test

Never delay treatment of suspected acute adrenal crisis to perform diagnostic testing. 1, 2, 3 If a patient presents with:

• Unexplained hypotension or shock 1, 2
• Collapse with gastrointestinal symptoms (vomiting, diarrhea) 2
• Vasopressor-dependent hypotension 2

Immediately administer IV hydrocortisone 100 mg and 0.9% saline infusion before any testing 1, 2. Blood can be drawn for baseline cortisol and ACTH before treatment if possible, but treatment must not be delayed 2.

Testing Patients on Corticosteroids

Do not attempt diagnostic testing in patients actively taking corticosteroids—the results will be falsely positive for adrenal insufficiency due to expected HPA axis suppression 2. Wait until corticosteroid treatment is discontinued and sufficient washout time has elapsed 2.

If you must treat suspected adrenal crisis but still want to preserve the ability to test later, use dexamethasone 4 mg IV instead of hydrocortisone, as dexamethasone does not interfere with cortisol assays 1, 2.

Interpreting Results in Context

Primary vs. Secondary Adrenal Insufficiency

The stimulation test confirms adrenal insufficiency but doesn't always distinguish the type 1:

• Primary adrenal insufficiency: Low cortisol with elevated ACTH (typically >300 pg/mL), often with hyponatremia and hyperkalemia 1, 2
• Secondary adrenal insufficiency: Low cortisol with low or inappropriately normal ACTH, aldosterone production remains intact 1, 2

Baseline Cortisol Can Sometimes Be Diagnostic

In acute illness with clear clinical suspicion 2:

• Basal cortisol <250 nmol/L (<9 μg/dL) with elevated ACTH is diagnostic of primary adrenal insufficiency 1, 2
• Basal cortisol <400 nmol/L with elevated ACTH raises strong suspicion 2

However, approximately 10% of patients with confirmed primary adrenal insufficiency present with normal basal cortisol concentrations but still fail stimulation testing, so a normal baseline does not exclude the diagnosis 7.