What is a late night salivary cortisol (LNSC) level?

Late Night Salivary Cortisol (LNSC): A Key Diagnostic Test for Cushing's Syndrome

Late night salivary cortisol (LNSC) is a highly sensitive and specific diagnostic test that measures cortisol levels in saliva during the evening hours to detect abnormal circadian rhythm of cortisol secretion, which is a hallmark of Cushing's syndrome. 1

What is LNSC and How Does It Work?

LNSC is based on the principle that:

• In healthy individuals, cortisol levels follow a circadian rhythm with highest levels in early morning and lowest at night
• Patients with Cushing's syndrome lose this normal circadian nadir of cortisol secretion 1
• Salivary cortisol reflects the free (biologically active) cortisol in plasma 2
• Samples are typically collected at bedtime or around midnight when cortisol should be at its lowest

Collection Method and Technical Aspects

• Non-invasive collection using commercially available saliva collection devices 2
• Samples should be collected at usual bedtime rather than at fixed midnight time 1
• At least 2-3 LNSC tests are recommended for accurate diagnosis 1
• Analysis can be performed using:
  • Immunoassay (better diagnostic performance for screening)
  • Mass spectrometry (better for detecting contamination with topical hydrocortisone) 3

Diagnostic Performance

LNSC demonstrates excellent diagnostic accuracy:

• Sensitivity: 92-100% 1, 4
• Specificity: 93-100% 1, 4
• Most specific test among the first-line screening tests for Cushing's syndrome 1

Advantages of LNSC

• Non-invasive and can be performed at home 5
• Not affected by changes in corticosteroid-binding globulin levels (unlike serum cortisol) 3
• Particularly useful for:
  • Women on estrogen therapy or during pregnancy 3
  • Monitoring for recurrence after treatment of Cushing's disease 3
  • Identifying cyclic Cushing's syndrome through sequential measurements 1
  • Screening large high-risk populations (e.g., patients with diabetes mellitus) 2

Limitations and Pitfalls

LNSC should not be used in:

• Patients with disrupted day/night cycles (e.g., night-shift workers) 1
• Samples potentially contaminated with topical hydrocortisone preparations 1

False positives may occur with:

• Physiologic activation of the HPA axis due to stress or illness 4
• Poor sleep quality
• Improper sample collection

Role in Diagnostic Algorithm

LNSC is recommended as a first-line screening test for Cushing's syndrome, along with:

• Overnight 1-mg dexamethasone suppression test (DST)
• 24-hour urinary free cortisol (UFC) 1

Recent evidence suggests that combining LNSC with late-night salivary cortisone measurement may further improve diagnostic accuracy, with a sensitivity of 95.2% and specificity of 100% 5.

Clinical Application

For suspected Cushing's syndrome:

1. Collect saliva samples at bedtime on at least two separate nights
2. Use laboratory-specific reference ranges for interpretation
3. Values above the upper limit of normal suggest Cushing's syndrome
4. Confirm with additional tests (DST, UFC) for definitive diagnosis

For subclinical Cushing's syndrome, LNSC has limited sensitivity (82%) and should not be used as the sole screening test 6.

LNSC has become an essential tool in the diagnostic workup of Cushing's syndrome due to its excellent performance characteristics and patient convenience.