Free Testosterone: Equilibrium Dialysis vs Calculation vs Direct Assay
How free testosterone is really measured, and mismeasured
Evidence grade 4 of 5, Strong. Read how the Atlas grades evidence.
The written rationale for this grade is still being prepared.
- FDA
- Not applicable (assay methodology)
Reference only. Not medical advice.
This is something to take to a clinician. The Atlas can tell you what is known about free testosterone: equilibrium dialysis vs calculation vs direct assay in general. It cannot tell you what is happening in your body, and the difference matters most for exactly the symptoms that bring people to this page.
Free testosterone can be measured three ways, of very different quality. Equilibrium dialysis (separating free from bound hormone across a membrane, then quantifying by LC-MS/MS) is the gold standard but is labor-intensive and offered by few labs. Calculated free testosterone (from total T, SHBG, and albumin via the Vermeulen equation) is the practical clinical standard and correlates well with dialysis. The widely-offered 'direct' analog free-testosterone immunoassay is cheap but notoriously inaccurate and is discouraged by the Endocrine Society, yet it appears on many routine panels.
The 'direct free T' your lab reports is often the least reliable number on the page.
What it is invoked to explain
Claims this idea is used to support. Whether it supports them is the rest of the entry.
How it works
Equilibrium dialysis physically partitions free from protein-bound testosterone before mass-spec quantification. The Vermeulen calculation models the binding equilibria of testosterone with SHBG and albumin from measured total T, SHBG, and albumin. The 'direct' analog immunoassay attempts to measure free hormone with a labeled analog but is confounded by binding-protein variation, making it unreliable.
What people get wrong
Often saidThe 'free testosterone' on my lab report is accurate
ActuallyIf it is a 'direct' analog immunoassay (common), it is the least reliable estimate. Calculated free T (Vermeulen) or equilibrium dialysis should be used for decisions.
Who profits, and who pushes back
Nobody is named here yet, because we have not done that work on this entry. Read that as unfinished. Somebody almost certainly sells this, and somebody almost certainly disputes it. Assume both until we have checked.
At a glance
- Gold standard
- Equilibrium dialysis + LC-MS/MS
- Clinical standard
- Calculated (Vermeulen equation)
- Avoid
- 'Direct' analog free-T immunoassay
What this entry does not yet cover
What follows is unfinished work on our side, not a finding about Free Testosterone: Equilibrium Dialysis vs Calculation vs Direct Assay. A short entry is not a safe entry.
- Timeline. We have not built the dated record of research and regulatory events.
- Cost and market. We have not documented what it sells for or who sells it.
- Supply chain. We have not mapped where it is made.
- Who uses it. We have not documented the user groups.
- What could change. No forward-looking summary yet.
- How it compares. We have not set it against the alternatives.
- Children and teenagers. We have not checked what is known about younger people.
- What can go wrong. We have not written the risk line for this one. Read the safety section below rather than treating its absence as reassurance.
- Where we land. We have not taken a position on this yet.
Sources
No sources recorded for this entry yet. Nothing on this page can be checked against a citation until that changes, so treat it as a summary awaiting its evidence.