The use of free cortisol index for laboratory assessment of pituitary-adrenal function

Area of Science:

• Biochemistry
• Endocrinology
• Immunoassay Development

Background:

• Accurate measurement of free cortisol is crucial for diagnosing pituitary-adrenal axis disorders.
• Existing methods for assessing free cortisol may have limitations.
• Cortisol binding globulin (CBG) plays a key role in regulating free cortisol levels.

Purpose of the Study:

• To develop and validate a time-resolved fluoroimmunoassay for measuring serum cortisol binding globulin (CBG).
• To evaluate the clinical utility of the Free Cortisol Index (FCI) as a screening tool for pituitary-adrenal axis disturbances.
• To compare the FCI with salivary cortisol measurements for estimating free cortisol concentrations.

Main Methods:

• Development of a microtitre plate, solid-phase, reagent excess, sandwich fluoroimmunoassay using polyclonal antiserum for both capture and labeled antibodies.
• Measurement of serum cortisol and CBG concentrations to calculate the Free Cortisol Index (FCI = [cortisol]serum/[CBG]serum * 100).
• Clinical investigation of FCI in diverse subject groups including healthy individuals, women on oral contraceptives, pregnant women, and patients with thyroidal or adrenal diseases, with comparisons to salivary cortisol.

Main Results:

• The developed fluoroimmunoassay for CBG was reliable and comparable to a commercially available kit.
• The Free Cortisol Index (FCI) was calculated using serum cortisol and CBG measurements.
• FCI values were assessed across various clinical groups and compared with salivary cortisol measurements.

Conclusions:

• The developed assay provides a reliable method for measuring serum CBG.
• The Free Cortisol Index (FCI) serves as a valuable screening tool for pituitary-adrenal axis disturbances.
• Utilizing the FCI can help avoid unnecessary further diagnostic testing compared to total cortisol measurements alone.