The test produces a colour change that depends on the level of oxygen in the sperm sample. At low levels of oxygen the sperm count is high. At a sperm count of 20 million sperm per millilitre, a couple usually has a 90% chance of conceiving within one year. Infertile men usually have less than 10 million motile sperm per millilitre.
The quality of the sperm, and the fertility of his partner matter too. A man with a low sperm count can still produce many offspring if his partner is fertile. Male infertility is much harder to cure than infertility in women.