How do I order a cortisol test through LabCorp?

How to Order a Cortisol Test Through LabCorp

To order a cortisol test through LabCorp, you should request a morning serum cortisol test as the first-line screening test, ideally collected between 7-9 AM when cortisol levels are at their peak.

Types of Cortisol Tests Available at LabCorp

Serum Cortisol Tests

• Morning serum cortisol: Collected between 7-9 AM
• 1 mg overnight dexamethasone suppression test (DST): Preferred screening test for autonomous cortisol secretion 1
  • Patient takes 1 mg dexamethasone at 11 PM
  • Serum cortisol measured at 8 AM the following morning
  • Interpretation:
    • <50 nmol/L excludes cortisol hypersecretion
    • 51-138 nmol/L suggests possible autonomous cortisol secretion

    • 138 nmol/L indicates cortisol hypersecretion

Other Cortisol Testing Options

• 24-hour urinary free cortisol (UFC): Requires complete 24-hour urine collection
• Late-night salivary cortisol (LNSC): Collected between 11 PM-midnight
  • Multiple collections (2-3) recommended for better accuracy 1

When to Order Each Test

• For screening adrenal incidentalomas: Order 1 mg DST as the preferred test 1
• For suspected Cushing's syndrome: Consider either DST, UFC, or LNSC based on clinical scenario 1
• For suspected adrenal insufficiency: Order morning serum cortisol paired with ACTH 1, 2

Specific Ordering Instructions

1. Complete a LabCorp test requisition form with the specific cortisol test needed
2. Include relevant clinical information on the requisition (suspected diagnosis, medications)
3. Specify collection timing (morning, late night, or 24-hour collection)
4. Provide patient instructions for proper test preparation:
   • For DST: Take 1 mg dexamethasone at 11 PM, blood draw at 8 AM
   • For 24-hour UFC: Complete collection instructions
   • For salivary cortisol: No eating/drinking 30 minutes before collection

Special Considerations

• Women on oral estrogen: DST may be unreliable; consider alternative tests 1
• Shift workers: DST may be preferred due to disrupted circadian rhythm 1
• Patients with suspected adrenal insufficiency: Order paired cortisol and ACTH 1
• Patients with suspected Cushing's syndrome: Multiple tests may be needed for confirmation 1

Interpreting Results

• Normal morning cortisol: 5-25 μg/dL (138-690 nmol/L)
• Adrenal insufficiency: Morning cortisol <250 nmol/L with increased ACTH is diagnostic 1
• Cushing's syndrome: Elevated late-night cortisol, failure to suppress with DST, or elevated 24-hour UFC 1

Common Pitfalls to Avoid

• Single measurements may be misleading due to cortisol's circadian rhythm 3
• Improper timing of sample collection can significantly affect results
• Failure to consider medications that affect cortisol levels (glucocorticoids, estrogens)
• Not measuring dexamethasone levels when false-positive DST is suspected 1

Remember that cortisol testing methods vary in specificity, with LC-MS/MS offering improved specificity compared to immunoassays, though both are available through LabCorp 4, 5.