PSA Doubling Time Calculator
PSA Doubling Time Calculator
Estimate the exponential doubling time between two PSA values, a strong predictor of metastasis after biochemical recurrence.
PSA Doubling Time
Two PSA values over timePSA 1.2 ng/mL rising to 2.4 ng/mL over 10 months
Formula
- PSA₁, PSA₂
- the earlier and later PSA values, ng/mL, ideally from the same assay
- months
- the interval between the two samples
- ln(2)
- 0.693 — the natural-log constant for exponential doubling
- negative or very large result
- means PSA fell or stayed roughly static over the interval — no doubling is occurring, which is not an error and is the reassuring outcome
Worked example
PSA 1.2 ng/mL rising to 2.4 ng/mL over 10 months
PSA₂ ÷ PSA₁ = 2.4 ÷ 1.2 = 2.000 — PSA exactly doubled over the interval measured
ln(2.000) = 0.693
10 × 0.693 ÷ 0.693 = 10.0 months
10 months falls in the 9–15 month band. A doubling time this short after treatment warrants imaging and specialist review; two values alone are a rough estimate, and at least three over a year give a far more reliable figure
Doubling time and risk after biochemical recurrence
| Doubling time | Risk |
|---|---|
| Under 3 months | Very high — often prompts systemic therapy |
| 3 – 9 months | High |
| 9 – 15 months | Intermediate |
| Over 15 months | Lower — often supports observation |
Reading a doubling time safely
Doubling time is one of the strongest predictors of metastasis and prostate-cancer-specific mortality after biochemical recurrence, and it is used directly in treatment decisions. A doubling time under 3 months carries a very high risk and generally prompts consideration of systemic therapy; over 15 months more often supports continued observation, particularly in an older man or one with significant comorbidity.
The calculation assumes simple exponential growth between two points, which is a strong assumption to hang a treatment decision on. At least three values collected over at least a year gives a far more reliable estimate than any single pair, because a two-point calculation can be badly distorted by prostatitis, recent instrumentation such as catheterisation or biopsy, or ordinary assay-to-assay variation — any of which can make PSA look like it is doubling in weeks when nothing has changed.
A negative or very large result is not an error. It simply means the second PSA was lower than, or barely different from, the first, so no exponential doubling is occurring over that interval — the calculation cannot express a falling PSA as a doubling time, and that absence of doubling is itself the reassuring finding worth reporting rather than discarding as a broken calculation.
Frequently asked questions
What is a concerning PSA doubling time?
Under 3 months carries a very high risk of metastasis and prostate-cancer-specific mortality and generally prompts consideration of systemic therapy. Over 15 months more often supports observation.
Why does my result show a negative or huge number?
That happens when the second PSA is lower than, or nearly equal to, the first. It means PSA is not doubling over the interval — not that the calculation has failed.
How many PSA values do I need?
At least three, spaced over at least a year, give a far more reliable estimate than a single pair, which can be distorted by prostatitis, recent instrumentation or assay variation.
Should the same laboratory measure every PSA used?
Ideally yes. Different assays calibrate slightly differently, and switching between them can introduce an apparent change in PSA that has nothing to do with the cancer.
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References
- Pound CR et al. Natural history of progression after PSA elevation following radical prostatectomy. JAMA. 1999;281(17):1591–97.
- Freedland SJ et al. Risk of prostate cancer-specific mortality following biochemical recurrence after radical prostatectomy. JAMA. 2005;294(4):433–39.