Sperm concentration determination between hemacytometric and CASA systems: why they can be different

Determination of sperm concentration is a critical component of semen analysis. Traditionally, the hemacytometer has been the standard for calibrating other technologies used to estimate sperm concentration, including photometry, Coulter counters, flow cytometry, and computer-automated semen analysis (CASA). Disposable capillary-loaded slides are commonly used in conjunction with most CASA systems currently in use. Questions have been raised regarding differences in sperm concentration measurements between CASA systems (using 20 microm disposable slides) and hemacytometry. This review explains that these differences are largely due to the Segre-Silberberg (SS) effect, which occurs during Poiseuille flow in thin, capillary-loaded slides. The SS effect can lead to errors in estimation of particle concentration, as demonstrated with latex beads and suspensions of human or porcine spermatozoa. The SS effect does not appear to have time to develop in the hemacytometer, which at 100 microm is considerably deeper than most disposable slides. Thus, hemacytometry, when properly performed, remains the gold standard for estimation of sperm concentration. When using thin (20 microm) slides with CASA systems, recognition of the appropriate compensation factor to adjust for the SS effect is critical for accuracy.