Delayed covering causes the accumulation of motile sperm, leading to overestimation of sperm concentration and motility with a Makler counting chamber.

According to the World Health Organization (WHO) manual, sperm concentration should be measured using an improved Neubauer hemocytometer, while sperm motility should be measured by manual assessment. However, in China, thousands of laboratories do not use the improved Neubauer hemocytometer or method; instead, the Makler counting chamber is one of the most widely used chambers. To study sources of error that could impact the measurement of the apparent concentration and motility of sperm using the Makler counting chamber and to verify its accuracy for clinical application, 67 semen samples from patients attending the Department of Andrology, West China Second University Hospital, Sichuan University (Chengdu, China) between 13 September 2023 and 27 September 2023, were included. Compared with applying the cover glass immediately, delaying the application of the cover glass for 5 s, 10 s, and 30 s resulted in average increases in the sperm concentration of 30.3%, 74.1%, and 107.5%, respectively (all P < 0.0001) and in the progressive motility (PR) of 17.7%, 30.8%, and 39.6%, respectively (all P < 0.0001). However, when the semen specimens were fixed with formaldehyde, a delay in the application of the cover glass for 5 s, 10 s, and 30 s resulted in an average increase in the sperm concentration of 6.7%, 10.8%, and 14.6%, respectively, compared with immediate application of the cover glass. The accumulation of motile sperm due to delays in the application of the cover glass is a significant source of error with the Makler counting chamber and should be avoided.