Free to Total PSA Ratio Calculator
Free to Total PSA Ratio Calculator
Calculate the percentage of free PSA, validated in the 4–10 ng/mL diagnostic grey zone to help avoid unnecessary biopsy.
Free to Total PSA Ratio
Free ÷ total × 100Free PSA 0.85 ng/mL, total PSA 6.2 ng/mL
Formula
- free PSA
- unbound PSA, ng/mL
- total PSA
- free plus protein-bound PSA, ng/mL
Worked example
Free PSA 0.85 ng/mL, total PSA 6.2 ng/mL
0.85 ÷ 6.2 = 0.13710
× 100 = 13.7% free PSA
Cancer probability by free PSA % (Catalona cohort, total PSA 4–10 ng/mL)
| Free PSA | Approximate probability of cancer |
|---|---|
| 0 – 10% | ~56% |
| 10 – 15% | ~28% |
| 15 – 20% | ~20% |
| 20 – 25% | ~16% |
| Over 25% | ~8% |
Why the free fraction falls with cancer risk
Prostate cancer cells produce proportionally more PSA bound to alpha-1-antichymotrypsin than benign tissue does, so as cancer risk rises the free, unbound fraction of PSA falls. In the classic Catalona multicentre series, men with a total PSA of 4–10 ng/mL and a free fraction below 10% had roughly a 56% probability of cancer on biopsy, against about 8% above 25% — a large enough separation to change management.
IMPORTANT: the ratio is validated only in that 4–10 ng/mL diagnostic grey zone with a normal digital rectal examination. It was derived and tested in that population specifically, and applying it to a total PSA of 2 ng/mL or 25 ng/mL is not supported by the evidence base, however tempting the arithmetic looks.
Free PSA is also analytically unstable. The sample must be separated and frozen promptly, because free PSA degrades faster than complexed PSA at room temperature, and a delayed spin lowers the free fraction independently of any biological change. 5-alpha reductase inhibitors such as finasteride and dutasteride also alter the ratio, so a free-to-total percentage taken on treatment cannot be interpreted against the untreated thresholds above.
Frequently asked questions
What free PSA percentage suggests cancer?
In the validated 4–10 ng/mL grey zone, below 10% carries roughly a 56% probability of cancer and above 25% roughly 8%, based on the original Catalona cohort.
Why does prostate cancer lower the free PSA percentage?
Cancer cells bind proportionally more PSA to alpha-1-antichymotrypsin than benign tissue, reducing the unbound, free fraction as cancer risk rises.
Can I use this ratio outside the 4–10 ng/mL range?
No. The ratio is validated specifically for a total PSA of 4–10 ng/mL with a normal digital rectal examination. It has not been shown to perform reliably outside that range.
Why might my free PSA result look wrong?
Free PSA degrades quickly if the sample is not separated and frozen promptly, and 5-alpha reductase inhibitors such as finasteride alter the ratio independently of cancer risk.
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References
- Catalona WJ et al. Use of the percentage of free prostate-specific antigen to enhance differentiation of prostate cancer from benign prostatic disease. JAMA. 1998;279(19):1542–47.
- Catalona WJ et al. Comparison of percent free PSA, PSA density, and age-specific PSA cutoffs for prostate cancer detection. Urology. 2000;56(2):255–60.