Do I need to fast before a cortisol level test?

Cortisol Testing and Fasting Requirements

No, you do not need to fast before a cortisol level test, but timing and other pre-test considerations are important depending on the specific cortisol test being performed. 1

Types of Cortisol Tests and Their Requirements

Blood Cortisol Test

• Fasting status: Not specifically required
• Timing: Morning cortisol levels (typically 8:00-10:00 AM) are preferred for baseline assessment as cortisol follows a diurnal pattern 1
• Considerations:
  • Morning cortisol values are affected by sex and pubertal status 2
  • Adults (Tanner stage 5) have higher morning cortisol levels than prepubertal children 2

Late-Night Salivary Cortisol (LNSC)

• Fasting status: Not required, but avoid eating, drinking, smoking, or brushing teeth 1-2 hours before collection 3
• Timing: Collected in the evening/night (typically between 11:00 PM-midnight)
• Clinical value: >90% sensitivity and 100% specificity for diagnosing Cushing's syndrome 1
• Considerations:
  • Avoid strenuous physical activity for 24-48 hours before sampling 3
  • Refrain from smoking/vaping/nicotine products for 4 hours before sampling 3
  • No alcohol consumption for 12 hours before sampling 3

24-Hour Urinary Free Cortisol (UFC)

• Fasting status: Not required
• Timing: Collected over a full 24-hour period
• Clinical value: Reflects overall cortisol production 1

Dexamethasone Suppression Test (DST)

• Fasting status: Not specifically required
• Timing: Dexamethasone taken at night, cortisol measured the following morning
• Considerations: Women on estrogen-containing oral contraceptives may have false positive results 1

Important Pre-Test Considerations

1. Physical and mental stressors:

   • Avoid strenuous exercise for 24-48 hours before testing 3
   • Mental stress can elevate cortisol levels 3
   • Cognitive assessments should be performed after cortisol sampling 3

2. Dietary factors:

   • Both overfeeding and fasting can increase circulating free cortisol levels 4
   • A 5-day fast can increase cortisol production rate by 1.8-fold 5
   • Even short-term fasting (72 hours) can markedly increase plasma cortisol 6

3. Diurnal variation:

   • Cortisol follows a strong circadian rhythm with highest levels in early morning 1
   • Fasting can delay peak cortisol concentrations to early afternoon 5
   • Afternoon cortisol responses may show greater adrenal sensitivity at certain time points 7

Common Pitfalls to Avoid

• False positives can occur in patients with severe obesity, uncontrolled diabetes, pregnancy, PCOS, psychiatric disorders, and alcohol use disorder 1
• False negatives can occur in patients with cyclic Cushing's syndrome and mild Cushing's syndrome 1
• Timing errors: Collecting samples at incorrect times can lead to misinterpretation due to cortisol's strong diurnal variation 3, 1
• Recent infections: Acute infections, especially respiratory tract infections within 2 weeks, can affect results 3

Key Takeaway

While fasting is not specifically required for cortisol testing, proper timing and avoiding factors that can artificially alter cortisol levels (exercise, stress, smoking, alcohol) are crucial for accurate results. The specific type of cortisol test being performed will determine the exact pre-test instructions that should be followed.