The gold standard Equilibrium Dialysis would be considered the most accurate assay when it come to free testosterone but if you live in a country which does not have access to such highly doubtful if you reside in the US then you would need to use/rely upon the calculated linear law-of-mass action Vermeulen (cFTV).
The only way to know where FT truly sits is to have it tested using the most accurate methods/assays (ED/UF) especially in cases of altered SHBG.
The gold standard would be Equilibrium Dialysis.
If you do not have access (highly doubtful if you reside in the US) to such then you would need to use/rely upon the go to calculated linear law-of-mass action cFTV which will give a good approximation but keep in mind it tends to overestimate FT.
As I have stated numerous times on the forum you always have the option of using/relying upon calculated FT which would be the linear law-of-mass action cFTV as it has already been validated twice (1st time was done using TT/SHBG assays no longer available) and was then eventually re-validated using current state-of-the-art ED method (higher order reference method) let alone more recently against CDCs standardized Equilibrium Dialysis assay.
Yes it tends to overestimate slightly but it is nothing to fret over!
*Calculated free T using high-quality T and SHBG assays has been considered the most useful for clinical purposes [99]. All algorithms suffer from some inaccuracies, including the variable quality of SHBG IAs [100], not replicating the non-linear nature of T-SHBG binding, different and inaccurate association constants for SHBG and albumin binding [101], and variable agreement with equilibrium dialysis results [99,100]. However, until further developments in the field materialize, the linear model algorithms [in particular, the most used Vermeulen equation [102]] appear to give, despite a small systematic positive bias, acceptable data for the clinical management and research[37,103]
hting its relevance in preventing misdiagnosis and overtreatment of male hypogonadism. Recent findings While there is consensus on measuring total T – comprising sex hormone-binding globulin (SHBG)-bound, albumin-bound, and free T – as a first step in diagnosing male hypogonadism, evidence...
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INTRODUCTION
Clinical practice guidelines on the diagnosis of male hypogonadism focus on clinical signs and symptoms of androgen deficiency, as well as biochemical assessment of low circulating testosterone (T). However, there is a longstanding debate, as well as a persisting controversy concerning biochemical assessment of serum T and in particular the use (and misuse) of free T. Free T, as advocated by the free hormone hypothesis, represents...
