What lab test should be ordered to check cortisol levels?

From the Guidelines

To check cortisol levels, either a 24-hour urinary free cortisol (UFC) test, a late-night salivary cortisol (LNSC) test, or a dexamethasone suppression test (DST) should be ordered, depending on the clinical scenario and local availability, as recommended by the most recent guidelines 1. When suspecting Cushing's disease, starting with either UFC and/or LNSC, or DST if LNSC is not feasible, is advised 1. The choice of test depends on various factors including patient convenience, potential disruption of the normal day/night cycle, and the need for longitudinal surveillance. For example, multiple LNSC tests may be easier for patient collection and are useful for patients with a normal day/night cycle, while DST may be preferred for shift workers or patients with disrupted circadian rhythm 1. Additionally, measuring dexamethasone level along with cortisol the morning after 1 mg dexamethasone ingestion can improve test interpretability 1. Some key points to consider when ordering these tests include:

• UFC: at least two or three 24-hour urine collections are advised to account for intra-patient variability 1
• LNSC: at least two or three tests are recommended, and sampling saliva at usual bedtime rather than at midnight could decrease false positive results 1
• DST: useful in shift workers, but may not be reliable in women treated with oral contraceptives 1. It is essential to follow the guidelines and consider the clinical context to choose the most appropriate test for checking cortisol levels.

From the Research

Lab Tests for Cortisol Levels

To check cortisol levels, the following lab tests can be ordered:

• 24-hour urinary free cortisol (UFC) test, which measures the amount of cortisol in the urine over a 24-hour period 2, 3
• Late-night salivary cortisol test, which measures the level of cortisol in the saliva at night 4
• Midnight serum or salivary cortisol test, which measures the level of cortisol in the blood or saliva at midnight 2, 3
• Dexamethasone suppression test, which measures the response of cortisol to a dose of dexamethasone 5, 6

Choosing the Right Test

The choice of test depends on the clinical suspicion and the patient's symptoms. For example:

• If Cushing's syndrome is suspected, a 24-hour UFC test or a late-night salivary cortisol test may be ordered 4, 2
• If the patient has symptoms of hypercortisolism, a midnight serum or salivary cortisol test may be ordered 2, 3
• If the patient has a history of Cushing's syndrome, a dexamethasone suppression test may be ordered to monitor the response to treatment 5, 6

Interpretation of Results

The interpretation of the results depends on the test and the patient's symptoms. For example:

• A high level of UFC in the urine may indicate Cushing's syndrome 2, 3
• A high level of cortisol in the saliva at night may indicate Cushing's syndrome 4
• A failure to suppress cortisol after a dose of dexamethasone may indicate Cushing's syndrome 5, 6