What does it measure?
The laboratory divides free PSA by total PSA and may express the result as a percentage. It uses two measured results rather than a third independent blood test. The ratio is intended to refine assessment in a suitable total-PSA range and clinical setting. [1]
Before reading the calculation
- Inputs
- Calculated from free PSA and total PSA.
- Use
- Its usefulness depends on the total PSA range and the clinical setting.
What can the result mean?
A lower free-to-total proportion can be associated with a higher likelihood of prostate cancer in the populations where the test was studied. A higher proportion can be reassuring within that assessment, but is not a guarantee. The percentage is not itself your personal percentage chance of cancer. [1]
Calculated from free PSA and total PSA.
- Calculation
- The laboratory divides free PSA by total PSA and may express the result as a percentage. It uses two measured results rather than a third independent blood test. The ratio is intended to refine assessment in a suitable total-PSA range and clinical setting.
- Use
- Its usefulness depends on the total PSA range and the clinical setting.
When is it useful, and how is it read?
Interpret it only with the total PSA range, compatible laboratory methods, age, and prostate assessment. It is a supporting piece when deciding whether further investigation is useful, rather than a stand-alone screening result. [1]
Reading the results together
| Marker | What it adds |
|---|---|
| Prostate-Specific Antigen (PSA), Total | The total measured PSA. |
| Prostate-Specific Antigen (PSA), Free | The portion not bound to proteins. |
| F/T PSA Ratio | The free portion relative to the total. |
The ratio adds context in selected cases; none of these confirms cancer on its own. [1]
What can affect it?
Both measurements should be compatible and preferably taken from the same sample. Age, prostate findings, total PSA, and laboratory method affect how the ratio can be used.
Myth: «The percentage is your chance of having cancer»
It is the percentage of PSA circulating in the free form. It can help estimate risk in the right setting, but it is not a direct percentage chance of cancer for you. [1]
Questions you might have
Is the ratio another blood substance?
No. It expresses free PSA relative to total PSA, as a fraction or percentage. [1]
Can it replace a biopsy or imaging?
No. It adds information for selected decisions; it does not diagnose cancer. [1]
What comes next?
Ask your clinician whether the ratio adds useful information before making decisions about further investigations. The decision should still include family history, symptoms, preferences, and any prior testing.
Is this ratio valid for my total PSA and does it meaningfully change the proposed follow-up?
The reference interval and units on your report are part of understanding your result. Understand units, reference intervals, and treatment targets.
Sources & references
Medical references & original research
Books we consulted
References that informed our explanations of the test and its context.
- Pagana KD, Pagana TJ, Pagana TN. Mosby’s Diagnostic and Laboratory Test Reference. 14th ed. Elsevier; 2019.
Practical-section sources accessed: .
For general health education. Interpreting your result and deciding on tests or treatment requires your health context and a qualified clinician.
