PSA kinetics: PSA doubling time and PSA density

What is measured: the rate of change of prostate-specific antigen and its concentration relative to gland size. How: doubling time is calculated by log-linear regression from at least three values over at least three months; density is PSA (ng/mL) divided by prostate volume (mL) on MRI or ultrasound, with 0.15 (0.10 with MRI) the usual threshold. Confounders: 5-alpha-reductase inhibitors halve PSA, prostatitis raises it. What a result changes: after radical prostatectomy a PSA of 0.2 ng/mL or more defines biochemical recurrence, and the EAU splits it into low and high risk by a doubling time of a year or less or a Gleason grade group of 4 or 5, which sets the urgency of salvage radiotherapy and whether hormone therapy is added; PSMA PET is positive in most men above 0.5 ng/mL; in non-metastatic castration-resistant disease a doubling time of ten months or less was the entry criterion for SPARTAN (apalutamide), PROSPER (enzalutamide) and ARAMIS (darolutamide), which lengthened metastasis-free and overall survival; a density above 0.15 with a PI-RADS 3 lesion tips towards biopsy, and density is an entry criterion for active surveillance (PRIAS 0.2 or less); in metastatic castration-resistant disease a 50 percent PSA fall (PSA50) is a trial endpoint, and a PSA under 0.2 at seven months of hormone therapy predicts long survival. Where it matters: biochemical recurrence, low-risk, non-metastatic castration-resistant and metastatic castration-resistant prostate cancer.