Intraindividual variation of PSA, free PSA and complexed PSA in a cohort of patients with prostate cancer managed with watchful observation

Objective: To determine the intraindividual variation of prostate-specific antigen (PSA) isoforms in prostate cancer patients managed conservatively with watchful observation. Methods: Patients with favorable clinical parameters (stage T1b-T2b NO MO, Gleason score less than or equal to 7, and PSA less than or equal to 15) were recruited to participate in a watchful observation program. Specimens were drawn for measurement of total (tPSA), free (fPSA) and complexed (cPSA) prostate-specific antigen isoforms. Total biologic variation and between-day analytical variation were used to calculate intraindividual variation. Results: Total variation for each isoform and two ratios were not greatly affected by the time window for measurements in the interval 6 months to 2.7 yr. Analytical variation made only a small contribution to the total biologic variation. Intraindividual variation for a 1-yr time interval for tPSA, fPSA, cPSA and the ratios of fPSA and cPSA to tPSA was, respectively, 21.6, 19.3, 25.4, 20.0 and 13.1%. The amount of change required for a significant difference between two readings (with 95% confidence) was, respectively, 59.8, 53.4, 70.4, 55.3 and 36.2%. Conclusions: There is a significantly higher intraindividual variation of cPSA (25.4%), and a significantly lower intraindividual variation of the ratio cPSA to tPSA (13.1%) compared to the other individual PSA isoforms and to the ratio of fPSA to tPSA. The amount of change required for a significant difference between two concentrations is large for all variables studied, but the lowest is the ratio of cPSA to tPSA (36.2%). These results have significance for diagnosis and monitoring of patients with prostate cancer. (C) 2002 The Canadian Society of Clinical Chemists. All rights reserved.