What does it measure?
Some PSA circulates without being attached to other proteins. This is called free PSA. Its main clinical value usually comes from comparing it with total PSA from the same blood sample, rather than treating the free value as a separate screening test. [1]
Before the test
- Sample
- A blood sample from a vein.
- Companion result
- Free PSA is interpreted with total PSA measured as part of the same assessment.
What can the result mean?
When total PSA falls in a range where the next step is uncertain, a lower proportion of free PSA can support a higher estimated chance of prostate cancer. A higher proportion may lower that estimate, but neither result proves or excludes cancer. [1]
When is it useful, and how is it read?
This can help when total PSA and the clinical assessment leave uncertainty about risk. Use compatible results from the same sample with age and examination findings; it does not replace an individualized decision about imaging or biopsy. [1]
What can affect it?
The usefulness of free PSA depends on the total PSA level, age, examination, and assay. Results from different laboratory methods are not automatically interchangeable. Prostate inflammation or recent procedures can complicate interpretation.
The proportion is usually more informative than the amount
Free PSA is usually interpreted as a share of total PSA. An isolated free value loses the comparison that makes the test useful in selected prostate assessments. [1]
Questions you might have
Can free PSA be interpreted on its own?
It is usually most useful as a proportion of total PSA in selected borderline results. [1]
Does a low free proportion confirm cancer?
No. It helps estimate risk; it does not replace the rest of the prostate assessment. [1]
What comes next?
Confirm whether your report includes the free-to-total percentage and whether it is valid for your situation. Your clinician can combine it with the broader assessment before deciding about repeat testing, imaging, or other investigations.
Is free PSA useful at my total PSA level, and what decision is it helping us make?
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.
