For a more accurate estimation of free testosterone (free T), it's generally recommended to use the Vermeulen calculation. Many labs, including Labcorp, use a direct method, which is a physical separation or analogue method. This method doesn't offer the same reliability as some other methods. In fact, Alex Vermeulen, Lieve Verdonck, and Jean M. Kaufman stated:

"Direct measurement of the FT concentration by an analog ligand immunoassay procedure is an attractive and simple alternative. Our study indicates, however, that the direct measurement of aFT by analog ligand RIA, although showing a generally good correlation with AFTC, is not a reliable index of FT; the aFT represents a variable fraction (20–60%) only of AFTC. This is confirmed by the fact that calculation of the SHBG binding capacity from aFT (aSHBG) yields values that are multiples of all values previously reported in the literature. Similar findings were reported by Wilke and Utley (7) as well as by Rosner (8). Our data show, moreover, that the aFT/AFTC ratio is SHBG dependent. Winters et al. (13) arrived at a similar conclusion."

Once you have your albumin measurement, you can calculate your free T using the provided calculator at https://www.issam.ch/freetesto.htm. To convert from ng/dl to pg/ml, simply multiply the result by 10.

The Vermeulen method was discussed in a 1999 paper by Alex Vermeulen, and its computation is grounded on the law of mass action. By determining the known association constants between testosterone, SHBG (Sex Hormone Binding Globulin), and albumin, they managed to calculate the theoretically predicted serum free testosterone concentrations. This method has been found to have a notable alignment with values measured using the equilibrium dialysis method, as long as testosterone, SHBG, and albumin measurements are taken using reliable assays.

There's an additional consideration regarding the role of albumin concentration when calculating free T. Variations in albumin concentration can influence the resulting free T values. Based on a study involving 30 subjects, even when there's a 25% change in albumin concentration, the difference in the calculated free T is relatively minor. Thus, for routine evaluations, free T can be calculated assuming a fixed albumin concentration of 43 g/L, unless dealing with specific cases like patients with significant plasma protein composition abnormalities, pregnancy, or certain illnesses where the precise albumin concentration should be accounted for.

You can read more about this research and its detailed findings in the provided academic article: https://academic.oup.com/jcem/article/84/10/3666/2660660.