Usually, a semen analysis is performed in order to assess sperm vitality. Fertilization potential and therefore semen quality rely on sperm motility. However, the motility test is not compulsorily performed in every semen analysis but only in case there are over 40% of immotile sperm.
When the motility of an sperm is zero, it is not possible to know whether this lack of motility is due to a failure in the ciliary machinery —the one allowing sperm motility — or to death spermatozoa. That is the reason why calculating the percentage of alive sperm in the ejaculate is necessary when there is a high level of immotile sperm.
Alive but immotile sperm may be used for that Assisted Reproductive Technology (ART) called ICSI. However, it would not be useful either to perform a conventional in vitro fertilization (IVF) or an artificial insemination (AI) since a sperm that does not “swim” properly may not be able to reach the egg.
Sperm motility test
The eosin test is the most common one used for the evaluation of sperm motility. It is also known as Williams Pollack test.
A eosin staining is realised to perform the motility test.
Dead sperm have multiple holes in their membranes, which means they will take up eosin and appear pinkinsh in colour. Normal live sperm will not be stained as they have intact membranes that prevent it to occur.
The hypo-osmotic swelling (HOS) test is also used for evaluation of sperm, although to a lesser extent. It consists of evaluating the functional integrity of the sperm’s plasma membrane of its tail. To perform it, semen is diluted in an hypoosmotic solution so that it enters the sperm cell. Normal live sperm will show a swelled tail because with the entrance of water. Thanks to this test, as happens with the eosin one, we are able to distinguish between dead sperm to those alive but immotile.
Results of the eosin stain test
After performing the stain test, the preparation is examined microscopically and 100 spermatozoa are recorded. Then, how many of these 100 are stained as well as how many have repeled the staining. This will allow us to count the percentage of dead and normal live sperm.
The World Health Organization (WHO) considers that the percentage of vital spermatozoa must be over 58%, whereas values below this figure indicate some kind of sperm alteration.
In order to perform an ICSI, it is important to have a low sperm count since only a single spermatozoon per ovum is used. Thus, although a percentage under 58% may indicate there is an anomaly, ICSI could still be carried out, since sperm count is not exceedingly low.
It should be taken into account however that a high number of dead sperm may indicate some other condition. This means that the remaining sperm, even if they are alive, may show some alteration. This could hinder pregnancy to go on to term even if the sperm ability to fertilize the egg is not compromised.
Necrospermia is a disease characterised by having more than 80% of dead sperm.