Pitfalls in interpreting prostate specific antigen velocity

Purpose: The concept of prostate specific antigen (PSA) velocity as an improved marker for prostate cancer detection is intriguing. However, before this concept is applied to individual patients several confounding parameters must be addressed. We determined the variability of serum PSA levels in men without prostate cancer. Materials and Methods: We reviewed data from a prostate cancer screening program, and determined inter-assay and individual variability of the serum PSA values for a 2-year followup period in 265 men clinically free of prostate cancer. Results: Our average inter-assay coefficient of variation was 7.5%. Therefore, we considered only PSA changes exceeding +/-15% as significant. Fluctuations in serum PSA occurred in 78% of the men during the observation period, and 12.5% had at least a single PSA increase exceeding 0.75 ng./ml. per year. Fluctuations were noted throughout the entire range of serum PSA levels but became progressively larger with an increasing mean PSA. Conclusions: The inter-assay variability must be considered when interpreting PSA velocity, Individual fluctuations in serum PSA dictate an observation period of at least 2 years before PSA velocity is considered abnormal.