Free PSA Unit Converter
Free PSA Unit Converter
Convert free PSA between ng/mL, µg/L and pmol/L — but a free PSA on its own has no interpretation. It means something only as a percentage of the total PSA, so no reference band is shown here.
Free PSA converter
Mass ⇄ molarFree PSA 0.85 ng/mL
Formula and conversion factor
pmol/L = ng/mL × 35.1741
% free PSA = (free PSA ÷ total PSA) × 100
- ng/mL = µg/L
- identical concentrations, so no conversion is needed between them
- 35.1741
- derived from a molecular mass of 28,430 Da for free, uncomplexed PSA. Molar reporting is not used clinically and no threshold is expressed in pmol/L
- no cut-off
- free PSA has no reference interval of its own — the interpretable quantity is the free fraction as a percentage of total PSA
- 4–10 ng/mL
- the total PSA grey zone in which the free-to-total ratio was derived and validated; outside it the percentage has not been shown to perform
Worked example
Free PSA 0.85 ng/mL
0.85 ng/mL = 0.85 µg/L — the two units are identical
0.85 × 35.1741 = 29.9 pmol/L
No band is shown, because free PSA alone has no cut-off
With a total PSA of 6.2 ng/mL this would be 0.85 ÷ 6.2 × 100 = 13.7% free PSA
Conversions
| ng/mL | µg/L | pmol/L |
|---|---|---|
| 0.50 | 0.50 | 17.6 |
| 0.85 | 0.85 | 29.9 |
| 1.00 | 1.00 | 35.2 |
| 2.00 | 2.00 | 70.3 |
Why this number needs a total PSA
| Question | Answer |
|---|---|
| Is there a normal range for free PSA alone? | No. It has no useful cut-off in isolation, which is why no band is shown on this page |
| What is interpretable? | The free fraction as a percentage of total PSA: (free ÷ total) × 100 |
| Where is that percentage validated? | In men with a total PSA of 4–10 ng/mL and a normal digital rectal examination |
| What does a low percentage suggest? | Below 10% carried roughly a 56% probability of cancer in the original Catalona cohort; above 25%, roughly 8% |
| What can make free PSA read falsely low? | Delayed separation and freezing of the sample — free PSA degrades faster than complexed PSA at room temperature |
| Does finasteride affect it? | Yes. 5-alpha reductase inhibitors alter both total and free PSA, so the untreated thresholds do not apply |
A number that cannot be read on its own
Free PSA is reported in ng/mL and in the numerically identical µg/L, so nothing needs to be calculated between them. A molar column in pmol/L uses a factor of 35.1741, derived from a molecular mass of 28,430 daltons for the free, uncomplexed protein. It is offered only for datasets that use it: molar reporting of PSA is essentially unheard of in practice and no clinical threshold is written in pmol/L.
This page deliberately shows no reference band, and that is the honest answer rather than an omission. A free PSA of 0.85 ng/mL is neither reassuring nor worrying, because the quantity that carries information is the free fraction expressed as a percentage of the total: free divided by total, multiplied by a hundred. The same 0.85 ng/mL is about 21 per cent of a total PSA of 4 ng/mL and about 8.5 per cent of a total of 10 ng/mL, and those two results point in opposite directions. Without the total PSA from the same sample, the free value cannot be interpreted at all.
The percentage itself is validated in one specific situation: men with a total PSA between 4 and 10 ng/mL and a normal digital rectal examination. That is the diagnostic grey zone in which the original Catalona work was done, where a free fraction below 10 per cent carried roughly a 56 per cent probability of cancer on biopsy against about 8 per cent above 25 per cent. Applying the same percentages to a total PSA of 2 or of 25 ng/mL is not supported, however easy the arithmetic looks.
Free PSA is also analytically fragile, and this is a common reason a result looks wrong. It degrades faster than complexed PSA at room temperature, so a sample that is not separated and frozen promptly gives a falsely low free fraction with no biological change behind it. 5-alpha reductase inhibitors such as finasteride and dutasteride alter PSA handling, so results on treatment cannot be read against the untreated thresholds. As with every marker on this site, the number is one input to a conversation with a clinician, not a verdict.
Frequently asked questions
What is a normal free PSA?
There is no useful reference range for free PSA on its own, which is why no band is shown here. The interpretable quantity is the free fraction as a percentage of total PSA, and that percentage is validated only when the total PSA is between 4 and 10 ng/mL.
How do I convert free PSA from ng/mL to pmol/L?
Multiply by 35.1741, a factor derived from a molecular mass of 28,430 daltons for free PSA. A free PSA of 0.85 ng/mL is 29.9 pmol/L. In practice PSA is never reported in molar units — ng/mL, identical to µg/L, is the working unit.
Why does free PSA need a total PSA to mean anything?
Because the same free PSA is favourable or unfavourable depending on the total. A free PSA of 0.85 ng/mL is about 21 per cent of a total of 4 ng/mL but only about 8.5 per cent of a total of 10 ng/mL, and those imply very different risks.
What free PSA percentage is concerning?
In the validated 4–10 ng/mL total PSA range with a normal digital rectal examination, below 10 per cent carried roughly a 56 per cent probability of cancer in the original Catalona cohort, and above 25 per cent roughly 8 per cent. This is a discussion to have with a urologist, not a result to act on alone.
Why might a free PSA result be misleading?
Free PSA degrades faster than complexed PSA at room temperature, so a delay in separating and freezing the sample lowers the free fraction artefactually. 5-alpha reductase inhibitors such as finasteride also change PSA handling, so the usual thresholds do not apply on treatment.
Related calculators
References
- Catalona WJ, Partin AW, Slawin KM, 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):1542–1547.
- Mottet N, van den Bergh RCN, Briers E, et al. EAU-EANM-ESTRO-ESUR-SIOG guidelines on prostate cancer — 2020 update. Eur Urol. 2021;79(2):243–262.
- Stephan C, Klaas M, Müller C, et al. Interchangeability of measurements of total and free prostate-specific antigen in serum with 5 frequently used assay combinations. Clin Chem. 2006;52(1):59–64.