Free PSA Percentage Calculator - Free-to-Total PSA Ratio

Free PSA percentage

The free PSA percentage calculator finds what share of a person's total prostate-specific antigen (PSA) is "free," meaning not bound to other proteins in the blood. Doctors use this ratio, alongside a total PSA test, to help judge prostate cancer risk when a biopsy decision is unclear.

What is PSA?

PSA is a protein made by cells in the prostate gland. A blood test measures how much of it is circulating. Most PSA in the blood is bound to other proteins. This portion is called complexed PSA. The rest floats freely and is called free PSA. Total PSA is the sum of both.

Prostate cancer cells tend to produce more of the bound form. Because of this, a lower percentage of free PSA is linked to a higher chance of cancer, and a higher percentage is linked to a lower chance.

How to calculate free PSA percentage

The formula

Free PSA percentage compares free PSA to total PSA, both measured in nanograms per milliliter (ng/mL):

$\text{Free PSA \%} = \dfrac{\text{Free PSA}}{\text{Total PSA}} \times 100$

Steps

1. Get the total PSA and free PSA values from a blood test.
2. Divide the free PSA value by the total PSA value.
3. Multiply the result by 100 to get a percentage.
4. Round to two decimal places.

Total PSA must be greater than zero. Free PSA cannot be negative, and it cannot be larger than total PSA, since free PSA is part of the total.

Worked example

A blood test shows a total PSA of 6.0 ng/mL and a free PSA of 1.2 ng/mL.

$\dfrac{1.2}{6.0} \times 100 = 20.00\%$

The free PSA percentage is 20.00%.

What the result means

Free PSA percentage is most useful when total PSA falls between 4 and 10 ng/mL. This range is often called the diagnostic gray zone, because a total PSA test alone cannot reliably separate benign prostate conditions from cancer there.

A widely cited 1998 study by Catalona and colleagues, published in JAMA, found the following approximate rates of prostate cancer at biopsy for men with total PSA in this range, grouped by free PSA percentage:

These figures come from one study population and are not exact odds for any individual. Age, prostate size, family history, and a digital rectal exam all factor into a doctor's judgment about biopsy.

A low free PSA percentage does not confirm cancer. Benign prostatic hyperplasia (an enlarged but non-cancerous prostate), prostatitis (inflammation of the prostate), and even recent ejaculation can lower it too.

When free PSA percentage is used

• Deciding on a biopsy: a higher free PSA percentage may support waiting instead of biopsying right away, since biopsies carry a small risk of infection and bleeding.
• Monitoring benign conditions: men with benign prostatic hyperplasia or prostatitis may be tracked over time rather than biopsied immediately.
• Active surveillance: men already diagnosed with low-risk prostate cancer may have PSA levels checked periodically.
• Research: the ratio is studied alongside newer markers, such as the Prostate Health Index and the 4Kscore test.

Frequently asked questions

What is a good free PSA percentage?

Above 25% is generally associated with lower prostate cancer risk. Below 10% is associated with higher risk. Values in between fall on a sliding scale, as shown in the table above.

Is free PSA the same as PSA percentage?

No. Free PSA is an amount, measured in ng/mL. Free PSA percentage is a ratio: free PSA divided by total PSA, shown as a percentage.

Can this calculator diagnose prostate cancer?

No. It only computes a ratio from two numbers. A doctor interprets that ratio together with total PSA, age, family history, exam findings, and sometimes imaging or biopsy.

Why does free PSA percentage matter mainly for total PSA between 4 and 10 ng/mL?

Below 4 ng/mL, cancer risk is already fairly low. Above 10 ng/mL, cancer risk is already high enough that most doctors recommend a biopsy regardless of the free PSA percentage. The ratio adds the most information in the range between.

What else can affect PSA levels?

Age, prostate size, urinary tract infection, prostatitis, recent ejaculation, and some medications can raise or lower PSA values, independent of cancer.

References

1. Catalona, W. J., et al. "Use of the percentage of free prostate-specific antigen to enhance differentiation of prostate cancer from benign prostatic disease: a prospective multicenter clinical trial." JAMA, vol. 279, no. 19, 1998, pp. 1542–1547.
2. "Prostate-Specific Antigen (PSA) Test." National Cancer Institute, cancer.gov/types/prostate/psa-fact-sheet.