Purpose: Because some patients show a surprising variation in serial serum prostate specific antigen (PSA) values, we determined the intra-individual or physiological variation in serum PSA by collecting sera 2 to 3 week apart without any prostatic manipulation.
Materials and methods: Because 4.0 to 10.0 ng./ml. PSA is the critical range for decision making, we asked all men with a PSA in this range to return 2 to 3 weeks later for a second measurement. Total serum PSA was determined by the Hybritech Tandem-R, automated Tosoh AIA-600 and Delfia section immunoassays. Free and complexed serum PSA was determined by the Delfia assays. Between assay variation (first blood specimen retested on a separate day with the second blood specimen) was compared to the physiological variation (first versus second blood specimens).
Results: Mean coefficient of variation (95% confidence limits) was 10.5% for between assay and 23.5% for physiological evaluations. The preferred analysis of ratio difference variation provided a factor of 0.138 (between assay) and 0.298 (physiological) for 95% confidence limits. Changes in free or complexed PSA were not the cause of physiological variation.
Conclusions: The intra-individual physiological variation is 2 to 3 times the between assay variation for sera drawn 2 to 3 weeks apart with a PSA of 4 to 10 ng./ml. A serum PSA of 4.0 ng./ml. can increase to 5.2 ng./ml. (4.0 x 0.298) and still be within the physiological variability for 95% confidence limits.