KLK3 and prostate carcinoma: To assess the rates of, and factors associated with, the uptake of PSA testing and opportunistic screening (that is, a PSA test in the absence of any symptoms) in England between 1998 and 2017, and to estimate the likely rates of pre-randomisation screening and contamination (that is, unscheduled screening in the ‘control’ arm) of the UK-based Cluster Randomised Trial of PSA Testing for Prostate Cancer (CAP).