What is the cortisol baseline?

From the Guidelines

A cortisol baseline test should be performed in the morning, between 7-9 AM, with a target cortisol level of < 1.8 μg/dL (50 nmol/L) after a 1-mg dexamethasone suppression test to accurately diagnose conditions like Cushing's syndrome or Addison's disease. To obtain accurate results, the test should be performed after an overnight dexamethasone suppression test, where a serum cortisol level is measured at 0800 h in the morning after 1 mg dexamethasone given between 2300 h and midnight 1. It is essential to avoid strenuous exercise, alcohol, and smoking for 24 hours before the test, and fast for 8-12 hours prior if it's a blood test. Certain medications like steroids, estrogen, and some seizure medications can affect results, so inform your healthcare provider about all medications you're taking.

Key Considerations

• The test helps diagnose conditions like Cushing's syndrome (high cortisol) or Addison's disease (low cortisol) by measuring the natural level of cortisol in the body, typically through blood, saliva, or urine samples.
• Normal morning cortisol levels typically range from 5-23 mcg/dL in blood samples, though reference ranges vary by laboratory.
• Multiple samples may be needed since cortisol follows a diurnal pattern, with highest levels in the morning and lowest at night.
• A recent study published in 2024 provides a comprehensive guideline for the diagnosis and management of pituitary adenomas in childhood and adolescence, including the diagnosis of Cushing disease 1.

Diagnostic Criteria

• The overnight dexamethasone suppression test is a reliable method for diagnosing Cushing's syndrome, with a sensitivity of 95% and specificity of 100% 1.
• A serum cortisol level of < 1.8 μg/dL (50 nmol/L) after a 1-mg dexamethasone suppression test is considered a normal response, while a level of ≥ 50 nmol/L (≥ 1.8 μg/dL) is indicative of Cushing's syndrome 1.
• The use of a corticotrophin-releasing hormone (CRH) test can also help confirm the diagnosis of Cushing disease, with a ≥ 20% increase in cortisol from baseline during the test being a diagnostic criterion 1.

From the Research

Cortisol Baseline

• The cortisol baseline, also known as the morning cortisol level, is an important indicator of adrenal function 2, 3, 4.
• A morning cortisol level of <100 nmol/l can indicate adrenal insufficiency, while a level of >300-500 nmol/l can indicate adrenal sufficiency 3.
• However, the exact cutoff values for morning cortisol can vary depending on the assay used and the specific clinical context 4.
• For example, a study using the Advia Centaur assay found that a morning cortisol level of >358 nmol/l could predict adrenal sufficiency with 100% specificity 4.
• Another study found that a basal cortisol value of >236 nmol/l could predict adrenal sufficiency with sensitivity 84% and specificity 71% 3.
• The 24-hour urine free cortisol (24 h UFC) test is also used to diagnose Cushing's syndrome, and has a sensitivity of 92.54% when used in combination with the low-dose dexamethasone suppression test (LDDST) 5.

Comparison of Tests

• The serum cortisol and 24 h UFC tests have different sensitivities and specificities in diagnosing Cushing's syndrome 5.
• The serum cortisol test has a higher sensitivity than the 24 h UFC test when used as a criterion for determining the diagnosis of Cushing's syndrome in combined LDDST 5.
• However, the 24 h UFC test has better sensitivity and specificity than the serum cortisol test in differentiating the etiology of Cushing's disease in combined high-dose dexamethasone suppression test (HDDST) 5.
• The overnight 1 mg dexamethasone suppression test (DST) and urinary free cortisol (UFC) tests are also used to screen for Cushing's syndrome, but each has its own pitfalls and limitations 6.