Extremely high values of prostate-specific antigen in patients with adenocarcinoma of the prostate; demonstration of the "hook effect".

We reviewed 721 consecutive samples submitted for measurement of prostate-specific antigen (PSA) over five months. We identified three patients with extremely high PSA concentrations: 650, 1840, and 3280 micrograms/L (their acid phosphatase activities were 3.2, 1337, and 2.8 U/L, respectively), and present case reports for the latter two. Serial dilutions of samples obtained from the patient with the highest PSA concentration indicated that the one-step Tandem-PSA assay gave falsely low values for high concentrations of PSA, an observation consistent with the phenomenon of the "hook effect." This effect was not observed when the sample was reanalyzed for PSA by a two-step procedure.