??A free PSA result cannot usually be understood properly without knowing the total PSA.
That is the most important point.
Total PSA measures the overall amount of prostate-specific antigen circulating in the blood. Free PSA measures the portion circulating without being attached to certain proteins. By comparing the two, it is possible to calculate the percentage of PSA that is free.
This percentage can sometimes add useful information when total PSA falls into an intermediate or borderline range. It does not diagnose prostate cancer.
A lower percentage of free PSA is associated with a higher probability of prostate cancer in selected men, while a higher percentage is more commonly associated with benign prostate conditions. There is considerable overlap between these groups, so the result must be considered alongside total PSA, age, symptoms, previous results and the wider clinical picture.
Free PSA vs Total PSA at a glance
Marker | What it measures | Main role |
Total PSA | Bound PSA plus free PSA | Main blood marker used in prostate assessment |
Free PSA | PSA circulating in an unbound form | Additional information when total PSA is difficult to interpret |
Percentage free PSA | Free PSA as a percentage of total PSA | May refine prostate-cancer risk assessment in selected men |
The clinically useful question is often not whether free PSA is simply “high” or “low”. It is:
What proportion of the total PSA is circulating freely?
What is free psa vs total psa?
PSA stands for prostate-specific antigen, a protein produced by prostate tissue. Some PSA circulates attached to proteins in the blood, while some remains unbound.
Total PSA includes both.
Total PSA = bound PSA + free PSA
A free psa test measures the unbound component. The two measurements are related, but they are not interchangeable.
Why does the percentage of free PSA matter?
Benign prostate enlargement and prostate cancer can sometimes produce similar total PSA concentrations. However, men with benign prostate enlargement tend, on average, to have a greater proportion of PSA circulating freely. Prostate cancer tends to be associated with a lower percentage of free PSA.
There is substantial overlap, so percentage free PSA cannot tell us “cancer” or “no cancer”. What it can sometimes do is refine the level of concern created by an intermediate total PSA result.
North West London Pathology describes percentage free PSA as an aid when distinguishing benign prostate enlargement from prostate cancer in selected men whose total PSA is in an intermediate range.
How is percentage free PSA calculated?
Free PSA ÷ Total PSA × 100
For example, if total PSA is 5.0 ng/mL and free PSA is 1.0 ng/mL:
1.0 ÷ 5.0 × 100 = 20% free PSA
The percentage is often considerably more informative than the free PSA concentration alone.
Why can a free PSA number be misleading on its own?
Imagine two people who both have a free PSA result of 0.5 ng/mL.
If total PSA is 2.0 ng/mL, percentage free PSA is 25%.
If total PSA is 8.0 ng/mL, percentage free PSA is 6.25%.
The free PSA concentration is identical, but the relationship with total PSA is completely different. That is why a standalone free PSA number cannot usually be labelled reassuring or concerning without the total PSA.
When is percentage free PSA most useful?
Percentage free PSA is best established as an additional marker when total PSA falls into an intermediate range—commonly around 4–10 ng/mL.
This is sometimes referred to as a PSA “grey zone” because benign prostate enlargement and prostate cancer can produce overlapping total PSA concentrations.
It should not be assumed that percentage free PSA has the same diagnostic value at every possible total PSA concentration.
How is percentage free PSA commonly interpreted?
Percentage free PSA | General association |
Above approximately 25% | Associated with a lower probability of prostate cancer |
Approximately 10–25% | Intermediate probability; other clinical factors become important |
10% or below | Associated with a higher probability of prostate cancer |
These are not diagnostic thresholds and are not NICE definitions of normal and abnormal PSA.
The American Cancer Society notes that some clinicians use percentage free PSA to help make decisions about further investigation when total PSA is between 4 and 10 ng/mL, although exact cut-offs and clinical practice vary.
Does a low percentage free PSA mean prostate cancer?
No. It means the probability may be higher than it would be with a higher percentage free PSA in a comparable clinical setting.
It is a risk marker, not a diagnosis. An abnormal total PSA or free-to-total PSA relationship may lead to clinical review and, where appropriate, prostate MRI before a decision is made about biopsy.
Can a higher percentage free PSA rule out prostate cancer?
No. A higher percentage is generally associated with a lower probability of prostate cancer when total PSA is in the intermediate range, but it does not reduce the probability to zero.
Can prostate cancer occur with a normal total PSA?
Yes. PSA testing is useful, but it is not perfect. Some prostate cancers occur without a markedly elevated PSA, while many people with raised PSA do not have prostate cancer.
The NHS PSA test guidance explains that PSA can be raised by several non-cancerous conditions and that a PSA result cannot determine on its own whether prostate cancer is present.
What else can raise total PSA?
Factor | Potential effect |
Benign prostate enlargement | More prostate tissue can produce more PSA |
Prostatitis | Prostate inflammation can increase PSA |
Urinary infection | May temporarily raise PSA |
Ejaculation | Can temporarily increase PSA |
Cycling or vigorous exercise | May temporarily influence PSA |
Prostate stimulation | Can affect the result |
Recent prostate procedure or biopsy | Can substantially alter PSA |
Some medicines | May lower or otherwise affect PSA interpretation |
How should you prepare for PSA testing?
Prostate Cancer UK recommends avoiding ejaculation and vigorous exercise, particularly cycling, for around 48 hours before the test, and advises avoiding anal sex or prostate stimulation for approximately one week beforehand.
Tell the clinician if you have recently had a urinary infection, catheterisation, prostate biopsy, urinary procedures or prostate surgery. Medicines such as finasteride and dutasteride can reduce PSA and materially affect interpretation.
Do not stop prescribed medication simply to alter a blood-test result.
Does a PSA blood test require fasting?
No. A PSA test does not normally require fasting. If it is measured as part of a larger profile, follow the preparation instructions for the complete panel.
How does age affect total PSA interpretation?
For people with symptoms that could indicate prostate cancer, current NICE guidance uses age-related PSA thresholds when considering suspected-cancer referral:
Age | PSA level above which referral should be considered |
Under 40 | Use clinical judgement |
40–49 | More than 2.5 µg/L |
50–59 | More than 3.5 µg/L |
60–69 | More than 4.5 µg/L |
70–79 | More than 6.5 µg/L |
80 and over | Use clinical judgement |
These are NICE referral thresholds for symptomatic people, not universal definitions of a safe or normal PSA result. Read the NICE guidance.
When is free psa testing worth considering?
Free psa testing can be particularly useful when total PSA is borderline or mildly elevated and additional information may help with risk assessment. It is not a replacement for total PSA.
Is free PSA useful if total PSA is normal?
Not routinely for everyone. Its best-established role is as an adjunct when total PSA is borderline or elevated.
Total PSA, Free PSA or both?
Question | Most relevant approach |
Initial PSA assessment | Total PSA |
Total PSA is borderline and additional context is needed | Total PSA + Free PSA |
Percentage free PSA is required | Both Total PSA and Free PSA must be available |
Previous Free PSA result without Total PSA | Total PSA is needed to calculate the percentage |
Symptoms or concerning PSA results | Clinical prostate assessment rather than relying on blood tests alone |
A standalone Free Prostate Specific Antigen test measures the free PSA component. If the aim is specifically to calculate and interpret the free-to-total PSA percentage, total PSA must also be available.
London Blood Tests' Prostate Profile includes both Total PSA and Free PSA, allowing those measurements to be considered together.
Booking PSA Testing in London
London Blood Tests provides private prostate blood testing through participating clinics, with eligible home and hotel blood collection also available.
Our Private Blood Tests in London guide explains appointments and blood collection.
Frequently Asked Questions
What does a free psa test measure?
A free psa test measures prostate-specific antigen circulating in an unbound form. It becomes particularly useful when compared with total PSA so that percentage free PSA can be calculated.
How is percentage free PSA calculated?
Free PSA ÷ Total PSA × 100.
What does a free PSA percentage above 25% mean?
When total PSA is in the intermediate range, a percentage above approximately 25% is generally associated with a lower probability of prostate cancer than a lower percentage. It does not rule out cancer.
What does a percentage free PSA below 10% mean?
A percentage of 10% or below is associated with a higher probability of prostate cancer when interpreted in the appropriate total-PSA range. It is not a cancer diagnosis.
Can free PSA be interpreted without total PSA?
Its standalone concentration can be reported, but the clinically useful percentage free PSA cannot be calculated without total PSA.
Does PSA testing diagnose prostate cancer?
No. PSA and percentage free PSA help assess risk. Diagnosis may require further clinical assessment and imaging, with biopsy used when tissue confirmation is necessary.
The Bottom Line
The most useful question is usually not “Is my free PSA high or low?” It is “What percentage of my total PSA is circulating freely?”
When total PSA is within an intermediate range—commonly around 4–10 ng/mL—the relationship between the two can sometimes provide additional information about prostate-cancer probability.
A lower percentage free PSA is associated with greater probability, while a higher percentage is generally associated with lower probability. Neither confirms or excludes cancer.
London Blood Tests offers a Free Prostate Specific Antigen test. Where the purpose is specifically to assess free psa vs total psa, both results must be available.