How to perform an adrenocorticotropic hormone (ACTH) stimulated morning (AM) cortisol test on a night shift worker?

How to Perform an AM Cortisol Test on a Night Shift Worker

For night shift workers, the overnight dexamethasone suppression test (DST) is the preferred method for cortisol testing rather than late night salivary cortisol (LNSC), as the latter should not be used in patients with disrupted circadian rhythms. 1

Understanding the Challenge

Night shift work significantly disrupts the normal circadian rhythm of cortisol secretion, which presents unique challenges when conducting cortisol testing:

• Normal cortisol secretion follows a diurnal pattern with peak levels in early morning and nadir at night
• Night shift workers experience altered cortisol patterns with blunted profiles 2, 3
• LNSC testing is specifically contraindicated in night shift workers due to disrupted day/night cycles 1

Recommended Testing Approach

Preferred Test: Overnight 1-mg Dexamethasone Suppression Test (DST)

1. Timing of administration:

   • Administer 1 mg dexamethasone between 2300h and midnight of the patient's subjective "evening" (before their sleep period)
   • This would typically be in the morning before they go to sleep after their night shift

2. Sample collection:

   • Draw serum cortisol at 0800h in the patient's subjective "morning" (after their sleep period)
   • This would typically be in the evening when they wake up

3. Interpretation:

   • Normal response: serum cortisol < 1.8 μg/dL (50 nmol/L)
   • Values above this threshold may indicate hypercortisolism

4. Enhancing accuracy:

   • Measure dexamethasone level concurrently with cortisol to verify adequate absorption and metabolism
   • Use laboratory-specific reference ranges for interpretation

Alternative Option: 24-hour Urinary Free Cortisol (UFC)

If DST is not feasible:

• Collect at least 2-3 complete 24-hour urine samples
• Collection should begin after voiding and include the first morning void of the following day
• Timing should be based on the patient's sleep-wake cycle, not clock time
• This test is less affected by circadian disruption as it measures total daily cortisol production

Important Considerations

• Adaptation period: Allow at least one week of consistent shift work before testing, as cortisol patterns may take time to stabilize 4

• Medication review: Check for medications that may affect dexamethasone metabolism:

  • CYP3A4 inducers (phenobarbital, carbamazepine) can cause false positives
  • CYP3A4 inhibitors (fluoxetine, cimetidine, diltiazem) can cause false negatives

• Potential confounders:

  • Oral estrogens can increase cortisol binding globulin levels
  • Chronic stress in shift workers may independently affect cortisol levels 5
  • Sleep duration affects cortisol awakening response 6

Common Pitfalls to Avoid

1. Do not use LNSC testing in night shift workers as it will yield unreliable results due to disrupted circadian rhythms 1

2. Do not rely on a single test result - confirm abnormal findings with a second test modality

3. Do not ignore chronotype (morning vs. evening preference) as it may influence cortisol patterns in shift workers 5

4. Do not test during transition periods between different shift schedules when cortisol rhythms are most disrupted

By following these guidelines, you can obtain more reliable cortisol measurements in night shift workers while accounting for their unique circadian physiology.