Preanalytical Determinants of Total and Free Prostate-Specific Antigen and Their Ratio: Blood Collection and Storage Conditions

Abstract

Prostate-specific antigen (PSA) is present in serum in several forms (1). About 70–90% of total PSA (t-PSA) is complexed with serum protease inhibitors, especially with α1-antichymotrypsin. About 10–30% are not bound to serum proteins (at least not with high affinity), and that fraction is called free PSA (f-PSA). Patients with prostate cancer exhibit a lower ratio of f-PSA to t-PSA (f-PSA%) than patients with benign prostatic hyperplasia (1). Subsequent studies have shown the clinical usefulness of f-PSA% in distinguishing between these two groups of patients (2). Whereas preanalytical, analytical, and biological factors of t-PSA changes have already been compiled (3)(4)(5), details on variation of f-PSA% are still lacking. The influence of preanalytical factors like blood collection, storage conditions, and freeze-thaw cycles on that ratio is of special interest because PSA as a nonurgent analyte is often quantified in batches after various storage periods (6). The purpose of the present study is to gain insight into these influencing factors on PSA fractions as measured by a widely used instrument and to lay down practical recommendations. AxSYM PSA and AxSYM Free PSA (Abbott Diagnostics), automated microparticle enzyme immunoassays, were used for measuring t-PSA and f-PSA, respectively (7). Each run was checked by three control sera for t-PSA (4.06, 14.6, and 45 μg/L) and f-PSA (0.38, 0.98, and 6.84 μg/L). The between-run CVs (n = 32) were 3.7–4.5% for t-PSA and 2.9–3.8% for f-PSA. The within-run CVs (n = 12) were 2.1–3.5%. The study included 18 patients with prostate cancer and 4 patients with benign prostatic hyperplasia. All procedures followed were approved by the Ethical Standard …