Free Androgen Index (FAI), Serum
| Free Androgen Index (FAI), Serum | | | |
| Number | | 146688 |
| CPT | | 84270; 84403 |
| Related Information | | Testosterone, Free and Weakly Bound Testosterone, Free (Direct), Serum |
| Synonyms | | FAI ; Testosterone:SHBG Ratio |
| Test Includes | | Testosterone; sex hormone-binding globulin (SHBG) |
| Special Instructions | | State patient's age and sex on the request form. |
| Specimen | | Serum |
| Volume | | 1 mL |
| Minimum Volume | | 0.6 mL (Note: This volume does not allow for repeat testing.) |
| Container | | Red-top tube or gel-barrier tube |
| Collection | | If a red-top tube is used, transfer separated serum to a plastic transport tube. |
| Storage Instructions | | Refrigerate |
| Causes for Rejection | | Plasma specimen |
| Reference Interval | | Male: - 20-29 years: 30-128
- 30-39 years: 24-122
- 40-49 years: 14-126
- Older than 49 years: 18-82
Female: - 20-49 years: 0.4-8.4
- Older than 49 years: 0.4-6.6
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| Use | | The free androgen index can be used to estimate physiologically active testosterone. |
| Methodology | | Immunochemiluminometric assay (ICMA) |
| Additional Information | | In most men and women, >50% of total circulating testosterone is bound to sex hormone-binding globulin, SHBG, and most of the rest is bound to albumin.1,2,3 SHBG-bound testosterone is not readily available for intracellular complex formation because of SHBG's high binding affinity for testosterone.2 Thus, testosterone-bound SHBG is considered to be biologically inactive. Albumin has a much lower binding affinity for testosterone but binds a significant portion of the total testosterone because albumin is present at much higher plasma concentrations than SHBG.2,4 The rapid dissociation of “weakly bound” testosterone from albumin, together with a relatively long transit time of albumin through target tissue capillary beds, result in the availability of essentially all albumin-bound testosterone for steroid-receptor interaction.4 The sum of the free- and albumin-bound testosterone is often referred to as bioavailable testosterone. The concentration of testosterone in the various free and bound forms is essentially a function of total testosterone concentration and the relative concentrations of SHBG and albumin. It can be predicted that increased SHBG will decrease the concentration of both free and bioavailable testosterone for a given total testosterone concentration. The free androgen index can be used to estimate physiologically active testosterone.2,3 This index is calculated as the ratio of total testosterone divided by SHBG (both expressed in the same units) and multiplied by 100 to yield numerical results comparable in free testosterone concentration.2,5,6,7 |
| Footnotes | | - Klee GG and Heser DW, “Techniques to Measure Testosterone in the Elderly,” Mayo Clin Proc, 2000, 75(Suppl):S19-S25.
- Wheeler MJ, “The Determination of Bioavailable Testosterone,” Ann Clin Biochem, 1995, 32(Pt 4):345-57 (review)
- Gronowski AM and Landau-Levine M, “Reproductive Endocrine Function,” Tietz Textbook of Clinical Chemistry, 3rd ed, Burtis CA and Ashwood ER, eds, Philadelphia, PA: WB Saunders Co, 1999, 1601-41.
- Manni A, Pardridge WM, Cefalu W, et al, “Bioavailability of Albumin-Bound Testosterone,” J Clin Endocrinol Metab, 1985, 61(4):705-10.
- Vermeulen A, Verdonck L, and Kaufman JM, “A Critical Evaluation of Simple Methods for the Estimation of Free Testosterone in Serum,” J Clin Endocrinol Metab, 1999, 84(10):3666-72.
- Blight LF, Judd SJ, and White GH, “Relative Diagnostic Value of Serum Non-SHBG-Bound Testosterone, Free Androgen Index and Free Testosterone in the Assessment of Mild to Moderate Hirsutism,” Ann Clin Biochem, 1989 (Pt 4):311-6.
- Wilke TJ and Utley DJ, “Total Testosterone, Free-Androgen Index, Calculated Free Testosterone, and Free Testosterone by Analog RIA Compared in Hirsute Women and in Otherwise-Normal Women With Altered Binding of Sex-Hormone-Binding Globulin,” Clin Chem, 1987, 33(8):1372-5
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