Bias and confounding in studies of sperm counts

Impairment of se:men quality has become a major topic in the public health debate the last 5 years. Some studies have shown a decline of sperm concentration during the past decades. Other studies, however, did not show a deterioration of sperm quality. All studies are characterised by the lack of comparable data on semen sample collection and on methodology used for semen analysis. Even studies performed in one centre do not guarantee the use of the same technique for sperm counting over years. It has also been well established that abstinence time, life style, drugs, smoking, alcohol abuse, stress and increase of scrotal temperature during fever and due to the presence of a varicocele may decrease sperm concentration. None of the studies have examined the potential impact of occupational conditions on reproduction. Nevertheless, certain professions are associated with reduced sperm quality. Furthermore, regional and ethnic differences may contribute to differences of sperm quality. Therefore, no definite answer can be given whether there is a time-related decline in sperm concentration.