How to measure cortisol levels in a patient?

From the Guidelines

Measuring cortisol in hospitalized patients should be done using a morning serum cortisol test, with a single measurement often sufficient, and normal values ranging from 5-25 μg/dL (138-690 nmol/L) 1.

Key Considerations

• For standard evaluation, a morning serum cortisol test is typically used, with blood samples collected in a red-top tube without additives and sent to the laboratory for analysis.
• If adrenal insufficiency is suspected, a cosyntropin stimulation test may be performed, administering 250 μg of synthetic ACTH intravenously and measuring cortisol levels at baseline, 30 minutes, and 60 minutes post-injection.
• A normal response shows cortisol rising to >18-20 μg/dL.
• For patients on steroids, medication should be held if possible for 24-48 hours before testing to avoid interference.
• Salivary cortisol testing is generally not used in inpatient settings due to logistical challenges, as noted in a study from 2017 1.
• 24-hour urine free cortisol collection may be ordered if Cushing's syndrome is suspected, requiring complete collection of all urine over a 24-hour period.

Diagnostic Approaches

• The overnight 1-mg dexamethasone suppression test (DST) can be used to diagnose Cushing's syndrome, with a serum cortisol < 1.8 μg/dL (50 nmol/L) at 0800 h in the morning after 1 mg dexamethasone given between 2300 h and midnight considered a normal response 1.
• Measuring dexamethasone concomitantly with cortisol, using laboratory-specific ranges of expected values, can reduce the risk for false-positive results 1.
• The low-dose dexamethasone suppression test (LDDST) can also be used, with a dose of 0.5 mg 6-hourly for 48 h, and serum cortisol level measured at 0,24, and 48 h 1.

Recommendations

• The most recent and highest quality study recommends using a morning serum cortisol test as the initial diagnostic approach 1.
• If Cushing's syndrome is suspected, the overnight 1-mg DST or LDDST can be used to confirm the diagnosis.
• Bilateral inferior petrosal sinus sampling for ACTH can be used to confirm a central source of ACTH excess, with a ≥2:1 ratio of central-to-peripheral ACTH before CRH or desmopressin and ≥3:1 ratio after CRH or desmopressin stimulation 1.

From the Research

Measuring Cortisol in Patients

To measure cortisol in patients, several methods can be used, including:

• Serum cortisol assays, which measure total cortisol and can be misleading in patients with altered serum protein concentrations 2
• Urine free cortisol, which is used to screen for Cushing's syndrome and correlates well with mean serum-free cortisol in conditions of cortisol excess 2
• Salivary cortisol, which reflects changes in unbound serum cortisol and offers a reliable alternative to measuring free cortisol in serum 2

Methods for Measuring Cortisol

The methods for measuring cortisol include:

• Automated immunoassays, which lack specificity and show significant inter-assay differences 2
• Liquid chromatography - tandem mass spectrometry (LC-MS/MS), which offers improved specificity and sensitivity 2
• Radioimmunoassay, which was used in a study to determine plasma cortisol levels 3

Diagnostic Criteria for Adrenal Insufficiency

The diagnostic criteria for adrenal insufficiency include:

• A mean basal morning cortisol level of < 110 nmol/L, which suggests adrenal insufficiency 3
• A mean basal morning cortisol level of > 300 nmol/L, which excludes the possibility of adrenal insufficiency 3
• A morning serum cortisol level of > 236 nmol/L, which predicts adrenal sufficiency with sensitivity 84% and specificity 71% 4
• A basal cortisol value of > 375 nmol/L, which predicts adrenal sufficiency with specificity 95% 4

Comparison of Efficacy between Serum Cortisol and 24 Hour Urine Free Cortisol

A study compared the sensitivity and specificity between serum cortisol and 24 hour urine free cortisol in combined dexamethasone suppression test in the diagnosis of Cushing syndrome, and found that:

• The sensitivity of serum cortisol of different cut off points after LDDST was 97.01%, 86.57%, 83.58% and 70.15% respectively 5
• The sensitivity of cutoff point of 24 h UFC <32 nmol in combined LDDST was 92.54% in the diagnosis of Cushing syndrome 5
• The sensitivity of serum cortisol <50 nmol/L was significantly higher than 24 h UFC<32 nmol (P<0.05) 5