PSA Doubling Time Calculator (PSADT)
Enter two PSA test results with dates (or the number of months between them) to calculate your PSA doubling time (PSADT) and velocity. PSADT measures how quickly prostate-specific antigen is rising, which helps clinicians assess the pace of prostate cancer after treatment. A result appears instantly, along with a risk category, step-by-step working, and a 24-month PSA growth projection.
What is PSA doubling time (PSADT)?
Prostate-specific antigen (PSA) is a protein produced by the prostate gland. After treatment for prostate cancer (surgery or radiotherapy), PSA ideally falls to very low levels. A subsequent rise signals that cancer cells may still be active, a situation called biochemical recurrence. PSA doubling time measures how quickly PSA is rising by calculating how long it takes to double. Shorter doubling times are associated with more aggressive disease and earlier progression to metastasis, so PSADT has become a widely used tool for guiding decisions about further treatment after primary therapy.
How is PSADT calculated?
The standard method uses the exponential (log-linear) model. First, the natural logarithm of each PSA reading is taken: ln(PSA2) - ln(PSA1). That difference is divided by the time between tests (in months) to give the slope. PSADT is then ln(2) / slope, where ln(2) is approximately 0.693. This is equivalent to asking: at the observed rate of exponential growth, how many months until PSA doubles? PSA velocity, the simpler absolute measure, is just (PSA2 - PSA1) / time and is expressed in ng/mL per month or year. Both metrics are clinically useful but measure different things: PSADT captures relative growth rate while velocity captures absolute speed of rise.
What do PSADT thresholds mean clinically?
In the landmark 1999 study by Pound and colleagues, 1,997 men were followed after radical prostatectomy. The overall median actuarial time to metastasis was 8 years from the point at which PSA first rose, and once metastatic disease appeared the median time to death was a further 5 years. Of all the cut-points the authors tested, a PSA doubling time of 10 months gave the strongest prediction of how soon metastasis occurred. The EAU has since drawn its formal line at 12 months: biochemical recurrence is classified as high risk when PSA-DT is 12 months or less (or the ISUP grade is 4-5) and low risk when PSA-DT exceeds 12 months with an ISUP grade below 4. A 6-month cut-off is widely used in practice as a further boundary, with PSADT under 6 months treated as high risk. PSADT under 3 months is considered very high risk and usually prompts urgent consideration of systemic treatment. Values above 12 months are generally regarded as favourable and often allow a period of active surveillance. Importantly, PSADT must be interpreted in the context of the absolute PSA level, time from primary treatment, histological grade, and the patient's overall health.
Limitations and important caveats
PSADT is a prognostic tool, not a diagnostic test. It should not be used for initial screening or diagnosis of prostate cancer. The calculation is only valid when PSA is rising on at least two measurements taken at the same laboratory, as assay differences can introduce apparent changes. Single-lab variability can shift PSADT estimates by 15-30%, so some guidelines recommend at least three serial PSA values over three months for a more reliable estimate. Benign prostatic hyperplasia (BPH), prostatitis, and other non-malignant conditions can also elevate PSA. Always discuss results with a urologist or oncologist who can interpret PSADT alongside the complete clinical picture.
PSA doubling time risk categories
| PSADT | Risk Category | Typical Clinical Action |
|---|---|---|
| < 3 months | Very High Risk | Urgent specialist review, consider systemic therapy |
| 3-6 months | High Risk | Close monitoring (1-3 monthly), specialist input |
| 6-12 months | Moderate Risk | Active surveillance, individualised treatment planning |
| > 12 months | Favourable | Watchful waiting with periodic PSA monitoring |
| 10 months (Pound et al.) | Strongest single cut-point | Best predicted time to metastasis in the 1999 cohort |
Risk stratification for biochemical recurrence after radical prostatectomy or radiotherapy. The 12-month boundary is the EAU low-risk/high-risk split (PSA-DT of 12 months or less is high risk, as is ISUP grade 4-5). The 3- and 6-month splits are conventional practice cut-points rather than guideline definitions.
Frequently asked questions
What is a normal PSA doubling time?
There is no single "normal" PSADT because the measure is only applied in the context of known or suspected prostate cancer recurrence after treatment. In healthy men, PSA rises slowly with age. In men being monitored after prostatectomy or radiotherapy, a PSADT above 12 months is considered favourable and is the EAU boundary for low-risk biochemical recurrence, while below 6 months is high-risk and below 3 months is very high-risk. For scale, the overall median actuarial time to metastasis in the Pound et al. landmark study was 8 years from the point PSA started rising.
How many PSA readings do I need for an accurate calculation?
This calculator uses two readings, which is the minimum needed and is sufficient for a clinical estimate. Some authorities recommend at least three readings over 3-6 months to reduce the influence of random PSA assay variability (which can be 15-30%). Memorial Sloan Kettering's multi-point method uses linear regression of log-PSA over all available time points, which reduces variability substantially. If you have more than two readings, consider using a multi-point tool for the most reliable estimate.
Can PSADT be used before prostate cancer treatment?
Pre-treatment PSA doubling time is sometimes used as a prognostic factor in men on active surveillance for low-risk prostate cancer, and a rapid pre-treatment PSADT may influence the decision to treat sooner. However, its primary and best-validated use is in the post-treatment biochemical recurrence setting after radical prostatectomy or radiotherapy. Interpretive thresholds differ by context, so always clarify with your clinician which setting applies to you.
What if my PSA is decreasing?
If your second PSA reading is lower than the first, PSA doubling time does not apply - the formula produces a negative number when PSA is falling. A declining PSA after treatment is generally a positive sign. Continue serial monitoring as your clinician advises; if PSA stabilises and then begins to rise, re-enter the two readings at that point to calculate a new PSADT.
Is PSADT the same as PSA velocity?
No - they measure related but different things. PSA velocity is the absolute rise in PSA per unit of time (for example, 0.5 ng/mL per year). PSADT is the relative doubling time, which depends on the starting PSA level. A man whose PSA rises from 0.1 to 0.2 ng/mL and one whose PSA rises from 5.0 to 10.0 ng/mL both have the same PSADT (same relative rate), but very different velocities. PSADT is generally preferred for tracking disease kinetics because it normalises for starting level, but both metrics are clinically informative.
What is biochemical recurrence?
Biochemical recurrence (BCR) means that PSA has risen above a defined threshold after primary treatment for prostate cancer, suggesting the cancer may be returning even if it cannot yet be detected on imaging. After radical prostatectomy, BCR is typically defined as PSA rising above 0.2 ng/mL on two consecutive readings. After radiotherapy, the Phoenix definition is a rise of 2 ng/mL above the PSA nadir (lowest point). BCR does not always progress to clinical metastasis, and PSADT is one of the key tools used to judge how quickly it is progressing.
Sources
- Pound CR et al. Natural History of Progression After PSA Elevation Following Radical Prostatectomy. JAMA 1999;281:1591-1597
- Prostate-Specific Antigen Working Group Guidelines on PSA Doubling Time. Journal of the National Cancer Institute 2009
- EAU Guidelines on Prostate Cancer - biochemical recurrence risk groups (PSA-DT of 12 months or less defines high-risk BCR)