Free testosterone can be measured three ways: direct immunoassay, calculated from total T and SHBG (Vermeulen equation), or equilibrium dialysis. They don't always agree — and the discrepancies matter for clinical decisions.
The Three Methods
| Method | Accuracy | Cost | Availability |
|---|---|---|---|
| Equilibrium dialysis | Gold standard | $100–$200 | Reference labs only |
| Calculated (Vermeulen) | Excellent correlation with dialysis | Included with total T + SHBG + albumin | Any lab that runs total T and SHBG |
| Direct immunoassay | Least accurate — can over/underestimate by 20–50% | $50–$100 | Most commercial labs |
The practical recommendation: avoid direct free T immunoassays when possible. Calculated free T (using the Vermeulen equation with measured total T, SHBG, and albumin) provides the best balance of accuracy, cost, and availability. Free online calculators exist (the ISSAM calculator is widely used) — just input your total T, SHBG, and albumin values.
The Bottom Line
Calculate, Don't Directly Measure
Calculated free testosterone from the Vermeulen equation is more accurate than most direct free T assays and more accessible than equilibrium dialysis. Ask for total T, SHBG, and albumin — then calculate. If your lab only offers direct free T immunoassay, interpret the result cautiously and use trending rather than absolute values.