What is the protocol for performing a cosyntropin (synthetic ACTH1‑24) test via intramuscular injection?

Cosyntropin Intramuscular Test Protocol

The standard intramuscular cosyntropin test involves administering 250 µg (0.25 mg) of cosyntropin reconstituted in 1 mL of 0.9% sodium chloride, with blood samples drawn at baseline and 30 or 60 minutes post-injection for serum cortisol measurement. 1, 2

Test Administration

Preparation and Dosing

• Reconstitute 0.25 mg cosyntropin powder with 1 mL of 0.9% sodium chloride injection 2
• Inspect the reconstituted solution visually for particulate matter and discoloration before injection 2
• Do not retain reconstituted product—use immediately 2
• Standard adult dose: 250 µg (0.25 mg) intramuscularly 1
• Pediatric dose (≤2 years): 125 µg (0.125 mg) often suffices 2

Blood Sampling Protocol

1. Baseline sample: Collect 6-7 mL blood in heparinized tube before injection 2
2. Inject 250 µg cosyntropin intramuscularly 2
3. Post-stimulation sample: Collect blood exactly 30 minutes or 60 minutes after injection 1, 2
   • Most subjects reach maximal cortisol response at 60 minutes with IM administration 3, 4
   • The 60-minute timepoint is preferred for diagnostic accuracy 3

Sample Handling

• Refrigerate both blood samples until laboratory processing 2
• If laboratory analysis cannot occur within 12 hours, separate plasma and refrigerate or freeze 2

Interpretation Criteria

Normal Response Thresholds

A peak stimulated cortisol ≥500 nmol/L (≥18 µg/dL) at 60 minutes excludes adrenal insufficiency in most clinical contexts 1. However, interpretation varies by assay and clinical context:

• Baseline cortisol should exceed 140 nmol/L (5 µg/dL) 1
• Normal incremental response: Approximate doubling of baseline level (if baseline is within normal range) 2
• Studies in healthy men show the lowest acceptable 60-minute cortisol is 16.3 µg/dL after IM administration 3
• Another validation study suggests post-ACTH cortisol <18.0 µg/dL indicates adrenal insufficiency 5

Important Caveats

Pre-test medication considerations:

• Patients on cortisone, hydrocortisone, or spironolactone should omit pre-test doses on testing day 2
• Estrogen-containing drugs may cause abnormally high baseline cortisol (elevated CBG) 2
• Cortisone/hydrocortisone may produce paradoxical response (decreased cortisol post-stimulation) 2

Conditions affecting interpretation:

• CBG deficiency, glucocorticoid resistance, or hypersensitivity can severely affect results 1
• Consider salivary cortisol measurement in patients with suspected CBG alterations 6

Practical Advantages of IM Route

The intramuscular route offers several clinical advantages:

• No venous cannulation required 4
• Can be performed without time constraints—not affected by diurnal variation 1
• Cost-effective and convenient screening test 4
• Validated correlation with 24-hour cortisol profiles 4

Comparison: IM vs IV Administration

While both routes are acceptable 1, there are pharmacokinetic differences:

• IM administration produces maximal cortisol response at 45-60 minutes 3, 4
• IV administration produces higher peak cortisol and more prolonged stimulation 7
• The 250 µg dose is highly validated for clinical practice regardless of route 1
• The 1 µg low-dose test does not provide better diagnostic accuracy than the 250 µg test 1

Critical Clinical Context

In acutely ill patients with suspected adrenal crisis:

• Draw baseline ACTH and cortisol before administering hydrocortisone 1
• Do not delay treatment to await test results 1
• Confirmatory cosyntropin testing can be performed after treatment with temporary cessation of glucocorticoids 1

The cosyntropin stimulation test remains the diagnostic gold standard for primary adrenal insufficiency, validated against the insulin tolerance test 1. The IM route at 250 µg with 60-minute cortisol measurement provides robust, practical diagnostic performance in routine clinical practice 3, 5, 4.