What are the considerations for interpreting salivary cortisol (cortisol measured in saliva) levels in diagnosing conditions like Cushing's syndrome or adrenal insufficiency?

Salivary Cortisol in Diagnosing Adrenal Disorders: Considerations and Limitations

Salivary cortisol is not recommended as the primary diagnostic test for adrenal insufficiency due to limited evidence of improved patient outcomes, practical collection challenges, and variable reliability compared to serum cortisol testing. 1

Salivary Cortisol: Basic Principles

• Salivary cortisol measures unbound (free) cortisol, which is the bioactive form of cortisol and represents approximately 10% of total serum cortisol 2
• Salivary cortisol follows the diurnal rhythm of serum cortisol and can be collected non-invasively 3
• Salivary cortisol is in equilibrium with free cortisol in plasma, making it theoretically useful for assessing adrenal function 4

Diagnostic Applications

For Cushing's Syndrome:

• Late-night salivary cortisol (LNSC) is a well-established screening test for Cushing's syndrome with sensitivity and specificity exceeding 90% 1, 5
• LNSC testing exploits the loss of normal circadian cortisol rhythm in Cushing's syndrome 1
• At least 2-3 LNSC samples are recommended for diagnosis 1
• LNSC is particularly useful for detecting cyclic Cushing's syndrome with periodic episodes of cortisol excess 1

For Adrenal Insufficiency:

• The Society of Critical Care Medicine and European Society of Intensive Care Medicine specifically recommend against using salivary cortisol rather than serum cortisol for diagnosing critical illness-related corticosteroid insufficiency (CIRCI) (conditional recommendation, very low quality of evidence) 1
• While stimulated salivary cortisol shows promise in some studies, with one reporting 90.7% sensitivity and 94.1% specificity using a cutoff of 13.2 nmol/L after ACTH stimulation 6, this approach has not been widely validated or incorporated into clinical guidelines

Limitations and Confounding Factors

• Salivary cortisol levels can be affected by multiple confounding factors:
  • Demographic variables: gender, age 1
  • Collection variables: time and site of sampling, saliva volume 1
  • Practical collection issues: insufficient saliva or blood contamination 1
• Implementation concerns are significant:
  • In one study, 19 of 57 patients were excluded due to inability to provide saliva samples, and 16 more were eliminated due to insufficient saliva or blood contamination 1
  • Salivary cortisol is typically measured by enzyme immunoassay, which may not be routinely available at most centers 1

Evidence Quality and Inconsistencies

• Studies comparing salivary cortisol to serum cortisol show inconsistent results:
  • In cirrhotic patients, free cortisol level was more strongly correlated with salivary than with serum total cortisol (Spearman coefficient 0.91 vs. 0.76) 1
  • However, in patients with septic shock, no significant difference was found between free serum cortisol and salivary cortisol levels, with good correlation (80%) between salivary and total serum cortisol 1
• The evidence demonstrating benefit of salivary cortisol over serum cortisol is extremely limited 1
• No study has demonstrated that using salivary cortisol to diagnose adrenal disorders leads to improved patient outcomes 1

Practical Recommendations

• For Cushing's syndrome screening:

  • Late-night salivary cortisol is appropriate and recommended 1, 5
  • Should be collected at usual bedtime rather than at fixed midnight time 1
  • Not suitable for night-shift workers or those with disrupted sleep-wake cycles 1

• For adrenal insufficiency diagnosis:

  • Serum total cortisol remains the standard approach 1
  • The high-dose (250-μg) ACTH stimulation test is recommended over low-dose testing 1
  • In patients with altered binding proteins (low albumin or CBG), calculating free cortisol using total serum cortisol and CBG measurements is preferable to salivary testing 1

Assay Considerations

• Multiple assay techniques are available for measuring salivary cortisol:
  • Radioimmunoassay (traditional method) 3
  • Enzyme immunoassay (commonly used) 1
  • Liquid chromatography-tandem mass spectrometry (LC-MS/MS) (improved specificity) 2, 3
• LC-MS/MS offers better specificity but is limited by lack of standardized assays and validated reference ranges 2
• Analytical sensitivity varies between assay methods, as does potential cross-reactivity with other steroids 3