Free cortisol index as a surrogate marker for serum free cortisol.
BACKGROUND
The biologically active component of a hormone is the unbound or free fraction. Changes in cortisol-binding protein could give misleading results if only total cortisol is measured for the interpretation of dynamic function tests.


METHODS
This study aimed to measure serum free cortisol using a steady-state gel-filtration method and then to evaluate the correlation between the serum free cortisol and the free cortisol index (FCI), defined as serum total cortisol/cortisol-binding globulin (CBG).


RESULTS
Forty-eight serum samples from healthy volunteers undergoing a short Synacthen test were analysed for total cortisol, free cortisol and CBG. The FCI correlated well with a previously established, but more complex, calculation of serum free cortisol (R = 0.98, P <0.001) and with measured serum free cortisol (R = 0.90, P < 0.001).


CONCLUSIONS
Free cortisol index is a reliable and user-friendly measure of serum free cortisol.
Department of Chemical Pathology, Charing Cross Hospital, London W6 8RF, UK. c.leroux@ic.ac.uk
Carel W
CW
le Roux
Sivajenani
S
Sivakumaran
Jamshid
J
Alaghband-Zadeh
Waljit
W
Dhillo
Wing May
WM
Kong
Michael J
MJ
Wheeler
D000328:Adult; D000893:Anti-Inflammatory Agents / Q000494:pharmacology; D015415:Biomarkers / Q000097:blood; D001774:Blood Chemical Analysis; D002352:Carrier Proteins / Q000097:blood / Q000378:metabolism; D002850:Chromatography, Gel; D005260:Female; D006801:Humans; D006854:Hydrocortisone / Q000097:blood* / Q000378:metabolism; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D011485:Protein Binding
D000893:Anti-Inflammatory Agents; D015415:Biomarkers; D002352:Carrier Proteins; C027091:cortisol binding globulin; D006854:Hydrocortisone