The free androgen index (FAI) test is a blood test that measures the level of androgens, such as testosterone, in the body. Androgens are hormones that play a critical role in male development and reproductive function. In women, androgens also play a role in reproductive health, but high levels can cause symptoms such as acne, excess hair growth, and menstrual irregularities. The FAI test measures the ratio of testosterone to sex hormone-binding globulin (SHBG), which is a protein that binds to testosterone and reduces its activity. By calculating the ratio of testosterone to SHBG, the FAI test can help determine whether there is a problem with androgen production or activity in the body. The test is commonly used to diagnose conditions such as polycystic ovary syndrome (PCOS), hypogonadism, or hirsutism. The FAI test is a simple and relatively non-invasive procedure that involves drawing a blood sample from a vein.