Sperm washing is a technique to prepare sperm for use in artificial insemination. Whether they are to be frozen or not, sperm should be subjected to this process. During the sperm-washing process, sperm are separated from the seminal fluid and from other constituents of semen such as chemicals and microorganisms. Some of the sperm-washing techniques also allow live sperm to be separated from dead or deformed sperm, a step which greatly increases the likelihood of a successful outcome of the insemination procedure.

Washed sperm are needed for intrauterine insemination of women hoping to get pregnant, because unprocessed semen contains prostaglandins which may cause severe cramping if the semen is introduced directly into the uterus. The washing process also removes bacteria and viruses from the sperm and helps avoid infections in the woman and even in the resulting child. As a result of the success of sperm washing in removing viruses, it’s now a real possibility for an HIV-positive man to have a child with an HIV-negative woman without risking that she and the child will be infected.

There are three main sperm-washing techniques, and the choice of technique depends on the type of sperm contained in the semen to be treated.

1. The swim-up method. This technique is suitable for normal semen samples. The semen sample is placed in a test tube and covered with a layer of washing medium. Sperm, which tend to swim upward, swim up into the medium, leaving behind the seminal fluid and dead sperm which are unable to swim. Only sperm which are sufficiently healthy and motile to swim up into the washing medium are collected, so this technique results in a high-quality sample. After being collected, the medium is centrifuged to concentrate the sperm, which are separated from the medium and then prepared for storage.

2. The centrifugation method. This method is used for semen samples containing sperm with low motility and for samples with a low sperm count. Low-motility sperm are less likely to be able to swim up into the washing medium without help, so the swim-up method is not suitable for sperm collection in this case. In the centrifugation method, the semen sample is mixed with the washing medium, and the mixture is centrifuged until the sperm separate out as a pellet. The pellet is collected from the centrifuged sample, suspended in more medium, and centrifuged again. This step can be repeated several times to ensure a pure sample.

3. The density gradient method. This is another centrifugation method, which is suitable for highly viscous samples and those with abnormal cells and debris. Washing media of different densities are layered in a tube, and the semen is added on top. Viable sperm are separated into the pellet after centrifugation, whereas the abnormal cells and debris are trapped in the different layers of medium. As with the regular centrifugation method, the density gradient process is repeated several times to ensure that the pellet eventually contains a pure sample.

Sperm samples are analyzed by the lab for abnormalities both before and after the washing process. The sperm-washing process thus results in a high-quality sperm sample with a good chance of successfully and safely fertilizing the eggs of a woman wishing to get pregnant.

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