Testosterone Total with Free Androgen Index

The Free Androgen Index (FAI), also referred to as the Free Testosterone Index (FTI), is an indirect method used to assess androgen status by estimating the concentration of circulating physiologically active testosterone. Bioactive testosterone consists of the unbound free fraction and the fraction weakly bound to albumin. Testosterone bound to sex hormone–binding globulin (SHBG), which accounts for more than 50% of circulating testosterone, is not considered biologically active due to the high binding affinity of SHBG.

Elevated SHBG concentrations reduce the proportion of bioavailable testosterone, even when total testosterone levels are within the reference range. Calculation of the FAI therefore incorporates both total testosterone and SHBG measurements to provide an estimate of androgen bioavailability. The FAI is calculated using the following equation: FAI = (Total Testosterone [ng/mL] × 3.47) / SHBG [nmol/L]

Refer to the individual laboratory procedures for Testosterone, Total and Sex Hormone Binding Globulin (SHBG) for additional analytical and clinical information.

The FAI is most commonly used in the evaluation of hyperandrogenic states, particularly in women with hirsutism or suspected androgen excess. In these patients, the FAI may correlate more closely with clinical manifestations than total testosterone measurement alone.

Store/transport at 15–30°C for ≤8 hours; 2–8°C for ≤48 hours; freeze at ≤ –20°C if delayed. Avoid repeated freeze–thaw cycles.

Hemolyzed or unseparated specimens.

Enhanced Chemiluminescence

This test includes reported measurements for Testosterone, Total and SHBG, with calculation and reporting of the Free Androgen Index.

• Himoto et al. Clinical efficacy of free androgen index, a surrogate hallmark of circulating free testosterone level, in male patients with HCV-related chronic liver disease. J Clin Biochem Nutr. 2018 Nov;63(3):238-245.
• Wilke et al. Total testosterone, free-androgen index, calculated free testosterone, and free testosterone by analog RIA compared in hirsute women and in otherwise-normal womenwith altered binding of sex-hormone-binding globulin. Clin Chem. 1987 Aug;33(8):1372-5
• Miller et al. Measurement of Free Testosterone in Normal Women and Women with Androgen Deficiency: Comparison of Methods. J Clin Endocrinol Metab. 2004 Feb;89(2):525-33.

Back to Reproductive Immunology