The hypoosmotic test is used to determine the vitality of the spermatozoa, i.e. the percentage of live and dead spermatozoa in the ejaculate. To do this, the spermatozoa are placed in contact with a hypotonic solution. In this way, it can be determined whether the sperm membrane is damaged or not, as the sperm would swell.
In addition, living spermatozoa will have a curled tail; whereas the tail of dead spermatozoa will be smooth.