When and how should the cosyntropin (synthetic ACTH) stimulation test be performed, and what cortisol response indicates normal adrenal function?

Cosyntropin Stimulation Test

The standard cosyntropin stimulation test should be performed using 250 µg administered intravenously or intramuscularly, with cortisol measured at baseline, 30 minutes, and 60 minutes, and a peak cortisol ≥500 nmol/L (18 µg/dL) traditionally indicates normal adrenal function, though newer specific cortisol assays require lower cutoffs of approximately 14-15 µg/dL. 1, 2

When to Perform the Test

The cosyntropin stimulation test is indicated for:

• Suspected primary adrenal insufficiency with clinical symptoms including volume depletion, hypotension, hyponatremia, hyperkalemia, fever, abdominal pain, hyperpigmentation, or hypoglycemia (especially in children) 1
• Confirmatory testing when morning cortisol is <140 nmol/L (5 µg/dL) with elevated ACTH (>2-fold upper limit of normal), unless results are absolutely unequivocal 1
• Screening for adrenal insufficiency in combination with other diagnostic tests 2

Critical Exception

In patients with severe adrenal insufficiency symptoms or adrenal crisis, immediately administer IV hydrocortisone at stress doses BEFORE awaiting test results. Draw baseline ACTH and cortisol before giving steroids, then treat. Confirmatory testing can be performed later after temporary cessation of short-acting glucocorticoids. 1

How to Perform the Test

Pre-Test Preparation

• Stop glucocorticoids and spironolactone on the day of testing (long-acting glucocorticoids may require longer cessation) 2
• Stop estrogen-containing drugs 4-6 weeks before testing, as they elevate cortisol-binding globulin and falsely increase total cortisol levels 2

Dosing

Adults: 250 µg (0.25 mg) cosyntropin IV or IM 2

Pediatric patients: 2

• Birth to <2 years: 125 µg (0.125 mg)
• 2-17 years: 250 µg (0.25 mg)

Administration Protocol

1. Obtain baseline serum cortisol
2. Administer cosyntropin as bolus injection (IV or IM)
3. Obtain cortisol levels at exactly 30 and 60 minutes post-injection 1, 2

The test can be performed at any time of day as results are not significantly affected by diurnal variation, though this flexibility should be balanced against clinical context. 1

Interpretation of Results

Traditional Cutoff (Older Polyclonal Immunoassays)

Peak cortisol ≥500 nmol/L (18 µg/dL) at 30 or 60 minutes indicates sufficient adrenocortical responsiveness 1

Revised Cutoffs for Newer Specific Assays

This is critically important: Newer monoclonal antibody immunoassays and LC-MS/MS require lower thresholds of 14-15 µg/dL depending on the specific assay used. 3, 4, 5

• Elecsys II: 14.6 µg/dL at 30 minutes 3
• Abbott Architect: 14.6 µg/dL at 60 minutes or 13.2 µg/dL at 30 minutes 5
• LC-MS/MS: 14.5 µg/dL 3

Using the historical 18 µg/dL cutoff with newer assays results in 17-18% false-positive diagnoses, leading to unnecessary glucocorticoid therapy. 3, 4, 5

Timing of Peak Response

• Most patients (54-65%) reach peak cortisol at 60 minutes 6, 7
• Some patients (especially overweight/obese) peak at 30 minutes 7
• Measuring both 30 and 60-minute values is recommended to avoid missing patients who peak later and prevent overdiagnosis of adrenal insufficiency 6, 7

Baseline Cortisol Predictive Value

• Baseline cortisol <2 µg/dL predicts subnormal stimulated response 3
• Baseline cortisol >11.1 µg/dL has 93% specificity for normal response (≥14.6 µg/dL), potentially obviating need for stimulation testing 8
• Baseline cortisol <1.5 µg/dL is always pathologic 4

Important Caveats and Pitfalls

Factors Affecting Interpretation

Conditions altering cortisol-binding globulin (CBG):

• Elevated CBG (falsely high cortisol): pregnancy, oral contraceptives 1, 9
• Low CBG (falsely low cortisol): cirrhosis, nephrotic syndrome, malnutrition, critical illness 1, 9
• Solution: Measure CBG levels concurrently if these conditions are present 2

Assay-specific considerations:

• Know which cortisol assay your laboratory uses
• Request assay-specific normative data and performance characteristics 1
• Different immunoassays show significant bias with reference limits ranging from 418-574 nmol/L (15.2-20.8 µg/dL) 1

Low-Dose (1 µg) Test

The 1 µg test provides no diagnostic advantage over the 250 µg test for primary adrenal insufficiency and should only be used when cosyntropin is in short supply. 1 The low-dose test may have higher sensitivity for secondary adrenal insufficiency, but this is not the primary indication for cosyntropin testing. 1

Diagnostic Accuracy

The high-dose (250 µg) test has 92% sensitivity for primary adrenal insufficiency and is validated against the insulin tolerance test as the diagnostic gold standard. 1

Confirming Primary vs. Secondary Adrenal Insufficiency

Measure plasma ACTH at baseline (can be obtained with baseline cortisol sample). In confirmed cortisol deficiency:

• ACTH >2-fold upper limit of normal indicates primary adrenal insufficiency 1
• ACTH >66 pmol/L (300 ng/L) represents maximum adrenal stimulation 1