Stimulation or irritation of the prostate can increase the levels of PSA produced. Examples include:
Urinary infection
Prostate cancer
Prostatitis (inflammation of the prostate, often caused by infection)
Benign prostatic hyperplasia (the prostate enlarges with age)
Vigorous exercise (especially cycling)
Prostate biopsies
Insertion of a urinary catheter
Ejaculation, anal sex or prostate stimulation
Some medications can decrease PSA levels, and give a lower result