Since the identification of prostate-specific antigen (PSA), continued technological advances have provided highly sensitive assays for its quantification. Given its lack of disease specificity, and its recent detection at low levels in an increasing number of non-prostatic tissues, PSA is far from being the perfect "tumour" marker (biological marker). However, the positive predictive value of PSA for assessing cancer risk makes PSA the most useful "tumour" marker for monitoring progression and response to treatment among patients with prostate cancer. Earlier detection through screening for elevated levels of PSA, while controversial, has been proposed as a way to decrease prostate cancer mortality. Haematogenous identification of PSA mRNA may provide stage-related prognostic information, and the use of ultrasensitive assays for PSA may permit earlier identification of residual or recurrent cancer, following treatment and the initiation of adjuvant therapy. Various PSA-related concepts, including the ratio of "free" PSA and complexes of PSA with the protease inhibitor, alpha(1)-antichymotrypsin, to total PSA, have been proposed and placed within diagnostic and management algorithms. Elevations of PSA in other irregularities of the prostate, notably in benign prostatic hyperplasia, and the increasing frequency and number of non-prostatic tissues, including those in women, expressing PSA, have implications for future immunoassays for PSA and strategies for immunotherapy using PSA-based monoclonal antibodies or vaccines, as well as for the molecular basis for its anomalous expression and physiological function(s).
