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.