(Direct) Free Testosterone With Total Testosterone
I looked into Equilibrium Ultrafiltration because I don't know what it was. It's a different method of testing testosterone levels in serum samples and is used when you want immediate results. For example, if you want to know the results of the blood test immediately after you're drawn, they'll run it through this machine that will provide quick results.
Investigation of equilibrium ultrafiltration as a means of measuring steroid-protein binding parameters
Abstract
The results presented in this study of the application of a novel technique of equilibrium ultrafiltration to the measurement of binding parameters for testosterone-BSA interaction, using the XM-50 Diaflo membrane, suggest that this method can provide, in experiments of short duration and in a manner amenable to robotization, valid analytical results of a high order of precision for the given conditions. However, further work is necessary to investigate the reasons for the approximately 25% higher binding affinity exhibited in this method as compared to equilibrium dialysis. Unlike the XM-50, the UM-10 exhibits both binding and rejection of the ligand, but it should nevertheless be possible in future work to take advantage of its low cut-off value by applying appropriate corrections for these phenomena.
Conclusion
We present a relatively simple UF coupled LC-MS/MS definitive method that measures serum free testosterone. The method is relatively fast, reliable and is suitable for the routine clinical laboratory practice.
The above article studied the effectiveness of this method and they declare it "suitable". It's jus' a buck more to get the "Direct" version; I wouldn't want my results skewed by accident.