Sperm viability is defined as the percentage of live sperm found in a semen sample. Viability is a crucial factor in fertility, as a high number of dead or non-moving sperm with lower the chance of natural conception. Sperm viability is a metric that can be measured through a semen analysis.

What is sperm viability?

Sperm viability refers to the proportion of live, membrane-intact spermatozoa in a semen sample, which is often used as an indicator of sperm quality. In cases of low sperm motility, when the sperm is unable to move quickly and efficiently toward the egg, doctors will usually test sperm viability to determine how many sperm are dead or alive. Necrozoospermia refers to the rare incidence in which all sperm in a fresh semen sample are dead.

The role of sperm viability in male fertility

Of the sperm released in a typical ejaculation, at least 75% should be alive and viable and at least 50% should be motile to reach and successfully fertilize an egg. But not all non-motile sperm are non-viable. In cases where motility is low but viability is high, doctors use sperm viability testing to identify which viable spermatozoa are appropriate for intracytoplasmic sperm injection (ICSI), a procedure that involves injecting a single live sperm directly into the center of a human egg.

Testing sperm viability

Sperm viability testing is performed by dye exclusion assays or hypoosmotic sperm swelling (HOS test).

In dye exclusion assays, dead sperm become stained with a dye, and live sperm are not. Trypan blue and Eosin Y stains are the most commonly used. However, these sperm do not typically survive testing, because they must be air-dried. This test simply provides doctors with an overview of sperm viability.

In a HOS test, live cells swell with water, particularly the sperm tail. Lab experts can then calculate the percentage of viable sperm. This assay does not damage or kill the sperm and is useful for identifying viable, non-motile sperm for ICSI.

Sperm viability after sperm freezing

While freezing sperm protects its quality, it is normal to see a slight decline in viability and motility after thawing cryopreserved sperm. Studies show that, on average, around 80% of sperm will survive the thaw. Those with normal, healthy sperm quality parameters produce an abundance of sperm — around 40–80 million sperm per ejaculate — and only 10 million sperm are needed for assisted reproductive technologies like intrauterine insemination, making the decline in viability clinically insignificant.

When using ICSI, only a single live sperm is needed for a successful fertilization, making sperm freezing an option even for those with a severely low sperm count (oligospermia).

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