Testosterone, Free, Direct

CPT: 84402
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Synonyms

  • Free Testosterone, Direct

Special Instructions

State patient's age and sex on the test request form.


Expected Turnaround Time

1 - 3 days



Related Documents


Specimen Requirements


Specimen

Serum


Volume

0.5 mL


Minimum Volume

0.3 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

Room temperature


Stability Requirements

Temperature

Period

Room temperature

14 days

Refrigerated

14 days

Frozen

14 days

Freeze/thaw cycles

Stable x3


Causes for Rejection

Plasma specimen; gross lipemia (may yield erroneously high free testosterone results)


Test Details


Use

Evaluate hirsutism and masculinization in women; evaluate testicular function in clinical states where the testosterone binding proteins may be altered (obesity, cirrhosis, thyroid disorders)


Limitations

A pediatric reference interval has not been established for this analyte. Gross lipemia may yield erroneously high free testosterone results.


Methodology

Direct analog enzyme immunoassay (EIA)


Reference Interval

See table.

Gender/Age (y)

Range (pg/mL)

Male

0 to 19

Not established

20 to 29

9.3−26.5

30 to 39

8.7−25.1

40 to 49

6.8−21.5

50 to 59

7.2−24.0

>59

6.6−18.1

Female

0 to 19

Not established

>19

0.0−4.2


Additional Information

The concentration of free testosterone is very low, typically <2% of the total testosterone concentration. 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 Routinely available assay methods used to measure total testosterone are not sensitive enough to accurately quantitate the free testosterone fraction directly. Free testosterone is estimated in this test by a direct, analogue immunoassay method. This assay uses a labeled testosterone analogue that has a low binding affinity for both SHBG and albumin but is bound by antitestosterone antibody used in the assay. Since the analogue is unbound in the plasma, it competes with free testosterone for binding sites on an antitestosterone antibody that is immobilized on the surface of the microtiter plate well.

Several authors have found that the analogue method has good correlation with equilibrium dialysis,1-4 but have found that the analogue results were only about one-fourth as high. Another group found that the analogue method produced results directly comparable to equilibrium dialysis without multiplication by a factor.5 More recently, Winters and coworkers have found the analogue method to correlates better with total testosterone levels than with bioavailable testosterone determined by the ammonium sulfate precipitation method.6 They suggested that the analogue-free testosterone results might be misleading in men with low SHBG concentration.6 Ooi suggested that the problems observed by Winters6 might, in large part, be resolved by simply using a more appropriate population-based reference interval.7 Vermeulen and coworkers found that the analogue-free testosterone method correlated well with free testosterone by equilibrium dialysis, but did not agree well with a free testosterone calculated from total testosterone and SHBG.3


Footnotes

1. Wheeler MJ. The determination of bio-available testosterone. Ann Clin Biochem. 1995 Jul; 32(Pt 4):345-357 (review). 7486793
2. Cheng RW, Reed MJ, James VH. Plasma free testosterone: Equilibrium dialysis vs direct radioimmunoassay. Clin Chem. 1986 Jul; 32(7):1411. 3719956
3. Vermeulen A, Verdonck L, Kaufman JM. A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocrinol Metab. 1999 Oct; 84(10):3666-3672. 10523012
4. Wilke TJ, 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 Aug; 33(8):1372-1375. 3608155
5. Déchaud H, Lejeune H, Garoscio-Cholet M, Mallein R, Pugeat M. Radioimmunoassay of testosterone not bound to sex-steroid-binding protein in plasma. Clin Chem. 1989 Aug; 35(8):1609-1614. 2758628
6. Winters SJ, Kelley DE, Goodpaster B. The analog free testosterone assay: Are the results in men clinically useful? Clin Chem. 1998 Oct; 44(10):2178-2182. Erratum: 1999 Mar; 45(3):444. 9761253
7. Ooi DS, Donnelly JG. More on the analog free-testosterone assay. Clin Chem. 1999 May; 45(5):714-716. 10223785

LOINC® Map

Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
144980 Testosterone, Free, Direct 2991-8 144981 Free Testosterone(Direct) pg/mL 2991-8

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