The Hypo Osmotic Swelling (HOS) test is an assay for assessing the viability of sperm and is a simple test which gives information about the sperm membrane integrity. Unlike the dye exclusion vitality test, the sperm are not fixed in the HOS test and thus it helps ICSI practitioners to correctly pick out viable spermatozoa from poor semen samples which show drastically compromised motility.
The basis of this test is the semi-permeability of the intact sperm plasma membrane and the osmotic gradient across it. Under hypo-osmotic conditions, there is a swelling of the plasma membrane due to the influx of water resulting in an expansion of the cell volume. In particular, the sperm tail is highly sensitive to swelling and curling. The swelling of the tail indicates membrane integrity and the curling indicates the integrity of the cytoskeletal structure of the sperm tail.