And yes I completely agree here. Here is the same plot above except with linear range:
Yes I understand why good advice is to use each labs free T result only in context of the method’s reported reference range. But if you are selling an expensive assay that claims to be an absolute measure of free Testosterone then you would hope the US reference laboratories would have better agreement than this on the upper reference range.
This type of harmonization process was recently completed for TT by LC/MS-MS and I hope they get there with FT as well:
Until then, I use calculated FT via Vermeulen and have seen enough to think it is pretty close (close enough) to Labcorp FT by ED. Secretly many of us FT nerds believe Quest has some issues with their FT method (there are both direct and indirect FT methods via ED that I won’t bore people with). And I won’t even bring up the Tru-T algorithm which even makes things more confusing for high SHBG guys.
If you want to have some fun use both of these calculators and share your FT numbers from both.