Free androgen index
ratio used to determine abnormal androgen status
Free Androgen Index (FAI) is a ratio used to determine abnormal androgen status in humans. The ratio is the total testosterone level divided by the sex hormone binding globulin (SHBG) level, and then multiplying by a constant, usually 100. The concentrations of testosterone and SHBG are normally measured in nanomols per liter. FAI has no unit.
FAI
=
100
×
(
total testosterone
SHBG
)
{\displaystyle {\text{FAI}}=100\times \left({\frac {\text{total testosterone}}{\text{SHBG}}}\right)}
The majority of testosterone in the blood does not exist as the free molecule. Instead around half is tightly bound to sex hormone binding globulin, and the other half is weakly bound to albumin. Only a small percentage is unbound, under 3% in males, and less than 0.7% in females. Since only the free testosterone is able to bind to tissue receptors to exert its effects, it is believed that free testosterone is the best marker of a person's androgen status. However, free testosterone is difficult and expensive to measure (it requires a time-consuming dialysis step), and many laboratories do not offer this service.
The free androgen index is intended to give a guide to the free testosterone level, but it is not very accurate (especially in males — see endocrine society commentary below).
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