Density gradient centrifugation allows us to separate a sperm sample depending on the spermatozoa's ability to go through several media, each with a different density.
Sperm with the best quality and motility will be able to reach the bottom of the tube, that is to say, to go through all the media in spite of their increasing density level. These sperm are the ones to use for the fertility treatment of choice.
Experts typically use two or three density gradients. The most common gradients are 45%, 80% and 90%.