Prior to the current availability of tests for measuring free testosterone, Carter et al used the ratio of total testosterone to sex hormone binding globulin (SHBG) to estimate the amount of bioavailable testosterone. This was termed the Free Androgen Index (FAI), but it also has been referred to as the Androgen Index (AI). It can be useful in evaluating hirsute women or if the free testosterone level is not available.
free androgen index =
= (total testosterone in nmol/L) * 100 / (SHBG in nmol DHT/L)
where:
• Nanjee and Wheeler use 1,000 instead of 100 in the equation.
Interpretation:
• In Carter et al original data (Figure page 263), the ratio for controls was from 1 to 5 (about half >= 2.8), while hirsute women showed a ratio from 2.8 to > 20 (9 patients <= 5).
• In Nanjee and Wheeler, about a third of the hirsute women overlapped with the range shown by normal control women (because of the use of 1000 instead of 100, the ratio values are about 10 times those shown by Carter et al.
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