The influence of sperm density on the motility characteristics of washed human spermatozoa.

To study the effects of sperm density on the results of computer-assisted semen analysis (CASA), 10 washed semen samples were diluted and measured with the CellTrak/S CASA system in a concentration range of 10-180 x 10(6) spermatozoa/ml. All sperm motility parameters were influenced to some extent by sperm density. The motility percentage was influenced significantly in 5 samples (P < 0.005), the straight line velocity in all samples (P < 0.0005 in 7 samples), the curvilinear velocity in 3 samples (P < 0.005), the linearity in 9 samples (P < 0.0005 in 6 samples) and the lateral head displacement in 9 samples (P < 0.005 in 6 samples). In general, the CellTrak/S data are influenced significantly if sperm density exceeds 50 x 10(6) spermatozoa/ml. The influence of sperm density on the motility parameters can be explained both by the accuracy of the CASA system and by actual changes in the motility of the spermatozoa. In the light of other published studies, it is concluded that sperm motility measurements with CASA systems should be assessed using 25-50 x 10(6) spermatozoa/ml, especially in studies concerning lateral head displacement and the linearity, as in sperm hyperactivation studies.