Free PSA vs total PSA: what the ratio tells you
If your PSA is only mildly raised, a second measurement from the same blood test can add useful context. Here is what each number means, what the ratio adds, and what to ask next.
In this article
If your PSA has come back mildly raised, someone may have mentioned a second test called free PSA. It can often be measured from the same blood sample, adds very little to the process, and can sometimes change how your result is read.
In a few cases, it can help you avoid a test you didn't need. Here is what total PSA and free PSA each measure, what the ratio between them adds, and what is worth asking about next.
What total PSA actually measures
PSA stands for prostate-specific antigen. It is a protein made by the prostate, and a small amount normally leaks into your bloodstream. Total PSA measures all of that protein in your blood at the time of the test. It is reported in nanograms per millilitre, written as ng/mL.
The important word is prostate-specific, not cancer-specific. Total PSA is raised by prostate cancer, but also by:
- a prostate that has grown larger with age (benign prostatic enlargement)
- a urine infection or inflammation of the prostate
- vigorous exercise or cycling in the previous two days
- ejaculation in the previous two days
- a recent catheter, prostate biopsy or other procedure
That is why a single raised number is a question, not an answer. What counts as "normal" also changes with age, and in the UK, GPs use age-specific thresholds. My articles on a PSA of 3, PSA of 5 and PSA of 10 explain what those levels mean.
Worth knowing: medicines for an enlarged prostate such as finasteride or dutasteride roughly halve the PSA reading after several months. If you take one, make sure whoever reads your result knows.
What free PSA measures
PSA travels in the blood in two forms. Most of it is attached, or bound, to other proteins. The rest floats on its own, and this unattached portion is called free PSA.
Total PSA counts both. Free PSA counts only the unbound part. The free-to-total ratio, often written as percentage free PSA, is simply the free PSA divided by the total, expressed as a percentage.
A worked example
- Total PSA: 6 ng/mL
- Free PSA: 0.6 ng/mL
- Free-to-total ratio: 0.6 ÷ 6 = 10%
Free PSA breaks down quickly once blood has been taken, so the sample needs to reach the laboratory promptly. That is one reason it isn't always added to a routine test.
Why the ratio matters when your PSA is only mildly raised
The ratio is most useful in what doctors call the grey zone, a total PSA of roughly 4 to 10 ng/mL. In this range, a raised PSA could be due to cancer or to an enlarged prostate, and the number alone can't tell you which. Cancer tends to produce relatively more bound PSA, so a lower proportion of free PSA points slightly more towards cancer.
These cut-offs are guides, not rules, and laboratories vary slightly. It is also worth being honest about the limits. Since prostate MRI became a routine step before biopsy, the free-to-total ratio is used less than it once was. It still earns its place in some situations, for example when your PSA has drifted up only a little and you and your specialist are deciding whether further tests are needed at all.
What a low ratio does and doesn't mean
A low ratio does not mean you have cancer. It means the balance of probability has shifted a little in your case. Plenty of men with a low ratio turn out to have nothing more than an enlarged prostate.
Nor does the ratio tell you how aggressive any cancer might be, or whether it would need treating. That information comes from an MRI, a targeted biopsy and the grade the pathologist reports, not from a blood ratio.
Read a low percentage as the reason your urologist wants a clearer picture, not as a verdict.
How the ratio sits alongside your other results
No single test stands alone. Your specialist will weigh the free-to-total ratio together with:
- Your PSA trend: a steady rise over time often matters more than any one reading.
- PSA density: your PSA compared with the size of your prostate on a scan. A large prostate naturally makes more PSA.
- Your MRI and PI-RADS score: the 1 to 5 grade a radiologist gives your MRI, where higher numbers mean a more suspicious area.
- Examination: how your prostate feels to your doctor.
- Your background: your age, general health and family history of prostate or breast cancer. Black men and men with a strong family history have a higher risk.
Taken together, these give a far more reliable picture than any single number. My article on the journey from a raised PSA to diagnosis shows how the steps fit together.
Questions worth asking next
Screenshot or print this list for your next appointment. A good consultation should leave you understanding your options, not just being told what happens next.
- What is my total PSA, and what is my free-to-total ratio?
- How has my PSA changed over time, and should it be repeated?
- Could anything have raised it temporarily, such as an infection, exercise or a medicine I take?
- What is my PSA density, and is my prostate enlarged?
- Would an MRI help before we decide about a biopsy?
- If a biopsy is needed, will it target a specific area as well as sample the rest of the prostate?
The bottom line
Free PSA and total PSA are two ways of looking at the same blood. Total PSA is the headline number. Free PSA, and the ratio it produces, adds a little more context when your PSA is only mildly raised, and can help you and your specialist decide whether further tests are needed.
It is one useful piece of the puzzle, not the whole puzzle.
