What is the protocol for ordering cortisol levels?

Protocol for Ordering Cortisol Levels

For suspected cortisol abnormalities, the appropriate testing protocol depends on the clinical scenario, with no single preferred diagnostic test for all situations. 1

Initial Screening Tests for Cushing's Syndrome

• 1 mg overnight dexamethasone suppression test (DST) is the preferred initial screening test for autonomous cortisol secretion when clinically appropriate 1
• For comprehensive evaluation, consider any of these three first-line tests:
  • 1 mg DST (overnight or 2-day low-dose) 1, 2
  • 24-hour urinary free cortisol (UFC) measurements (at least 2-3 collections) 1, 2
  • Late-night salivary cortisol (LNSC) (at least 2-3 samples) 1, 3

Test Selection Based on Clinical Scenario

• For adrenal incidentalomas: All patients should be screened with 1 mg DST 1
• For suspected Cushing's syndrome:
  • Start with DST, UFC, and/or LNSC depending on local availability 1
  • Multiple LNSCs may be easier for patients to complete 1
  • If an adrenal tumor is suspected, start with DST 1

Special Considerations for Test Selection

• Shift workers or disrupted circadian rhythm: DST is preferred 1
• Women on oral estrogen: DST may be unreliable; consider measuring dexamethasone level if false-positive suspected 1
• Suspected malabsorption: Serum or salivary cortisol day curve monitoring may be useful 1

Timing and Collection Protocols

• Morning serum cortisol:

  • Best collected between 6:00-8:00 AM 3
  • Values <83 nmol/L are diagnostic for central adrenal insufficiency 4
  • Values >414 nmol/L exclude central adrenal insufficiency 4
  • Values between 83-414 nmol/L require stimulation tests 4

• 24-hour urinary free cortisol:

  • Collect at least 2-3 specimens to account for variability 1
  • Ensure complete 24-hour collection for accuracy 5, 6

• Late-night salivary cortisol:

  • Collect at 10:00 PM for optimal diagnostic value 3
  • Multiple samples (2-3) recommended due to potential variability 1

• Aldosterone/renin ratio (if primary aldosteronism suspected):

  • Collect in the morning after patient has been out of bed for 2 hours 1
  • Patient should be seated for 5-15 minutes before collection 1
  • Ensure patient is potassium-replete 1
  • Substitute interfering medications if possible 1

Laboratory Methods

• Both immunoassays and liquid chromatography-tandem mass spectrometry (LC-MS/MS) are acceptable 5
• LC-MS/MS offers improved specificity and sensitivity but requires validated cut-offs 5, 6
• For salivary cortisol, LC-MS/MS is the method of choice 5

Interpretation Guidelines

• For 1 mg DST:

  • Cortisol <50 nmol/L excludes cortisol hypersecretion 1
  • 51-138 nmol/L suggests possible autonomous cortisol secretion 1

  • 138 nmol/L indicates evidence of cortisol hypersecretion 1

• For 2-day LDDST:

  • Cortisol suppression below 50 nmol/L (1.8 μg/dL) is considered normal 2
  • Post-LDDST cortisol ≥50 nmol/L is diagnostic for Cushing's syndrome 2

• For late-night salivary cortisol:

  • Age-specific cut-offs should be used 3
  • For adults (21-60 years): values >1.9 μg/L strongly suggest Cushing's syndrome 3

Common Pitfalls

• Medications affecting dexamethasone metabolism can cause false results:
  • CYP3A4 inducers (phenobarbital, carbamazepine) may cause false positives 2
  • CYP3A4 inhibitors (fluoxetine, cimetidine) may cause false negatives 2
• Altered serum protein concentrations can affect total serum cortisol measurements 5
• Incomplete 24-hour urine collections can lead to inaccurate UFC results 6