How this calculator works
The two-point model assumes exponential change between positive PSA values. Clinical PSA doubling-time assessment usually uses several samples over an adequate interval; two points are sensitive to assay variation and timing. Falling PSA is reported as a halving time, while unchanged PSA has no finite doubling time. Velocity uses a 365.25-day year. These calculations cannot determine cancer status or treatment response alone. The result is an estimate for information only and is not medical advice.
Formula
Log slope = [ln(PSA2) - ln(PSA1)] / elapsed days. Doubling time = ln(2) / positive log slope. For negative slope, halving time = -ln(2) / slope. PSA velocity = (PSA2 - PSA1) / elapsed years. Inverse model: PSA2 = PSA1 × 2^(elapsed/doubling time).
Worked example
A rise from 2 to 4 ng/mL over 180 days doubles once, so doubling time is 180 days. A rise from 2 to 8 over 180 days doubles twice, giving 90 days. A fall from 4 to 2 over 180 days has a 180-day halving time.
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Frequently asked questions
Why is no doubling time displayed for unchanged PSA?
The logarithmic slope is zero, so there is no finite time to double in this model.
Is this equivalent to regression over many samples?
No. A clinical calculation may fit a line to the logarithms of several PSA measurements; this page is explicitly a two-point calculation.
Can I use PSA reported as zero?
No. The logarithm needs a positive value. A value below an assay detection limit is not an exact zero and should not be substituted arbitrarily.