Quest has two testosterone tests used to calculate free testosterone, more specifically if your doctor relies on this test in making an evaluation he in in error at least when it comes to your free testosterone
Free and Total Testosterone
â?? Total: LC/MS/MS
â?? Percent free: equilibrium dialysis
â?? Free: calculated based on total and percent free
â?? Aliases: testosterone index, dialyzable testosterone
â?? CPT Codes:* 84403, 84402
The problem with this test stems from the fact that it is designed as way to detect free testosterone in people with extremely low levels, where as the other test Quest uses is made for people with normal levels. The reason this test can detect levels better is because for free testosterone, Quest uses the equilibrium dialysis method to determine the percent free which correlates more accurately in extremely low people than by just basing it off of shbg, albumin and total test algorithm used in their other test.
The test results itself are not inaccurate but the range Quest uses for free testosterone only scales up to 155 pg or 15.5 ng. However, for most normal men the top of the range for free testosterone before they begin to exhibit negative side effects is around 224 pg or 22.4 ng. This is the much more accurate range Quest uses in their other test. Both tests will also yield very similar results.
Going more in depth with how this ultra sensitive free testosterone test works, basically in lamen terms Quest determines the percentage of free testosterone using equilibrium dialysis out of the amount of total testosterone in your blood. They then take that number turn it into a fraction and multiply it by your total testosterone amount then multiply that number by 10 to go from ng to pg. So for example if your total testosterone was 600 and you had roughly 2.7 percent free you would be above their reference range of 155 pg since your levels would be 162 pg, but you would be squarely a little above the middle in their other test.
Furthermore, in order for someone lets say to be above the high range on free testosterone at 600 total testosterone (aka above 22 pg) they would have to have a shbg of around 2 which is physiologically impossible. Also anyone with a total testosterone level of 800 and above and a free testosterone percentage of as low as 2 percent would be above this scale. (in literature most males are reported to have around 1 to 3 percent free).
Here is a link showing their different scales. Here is the other correct test your doctor should order
Free, Bioavailable, and Total Testosterone
â?? Total: LC/MS/MS
â?? Free: calculated based on constants for the binding of testosterone to SHBG and albumin
â?? Bioavailable: calculated based on constants for the binding of testosterone to SHBG and albumin
â?? SHBG: extraction, chromatography, radioimmunoassay
â?? Albumin: spectrophotometry
â?? Aliases: free, weakly bound, and total testosterone
â?? CPT Codes:* 84403, 84270, 82040