World Health Organization reference values for human semen characteristics
The ‘WHO manual for the examination of human semen and sperm[semen]-cervical mucus interaction’ (WHO, 1987, 1992, 1999) is widely used as a source of standard methodology for laboratories engaged in semen analyses. However, the interpretation and application of previous WHO ‘normal’ or ‘reference’ values for semen parameters used thus far have limitations, since the data were derived from imprecisely defined reference populations and obtained from laboratories with unknown comparability with respect to analytical methodologies. These values were limited by the lack of available data on semen variables in recent fathers, and did not define true reference ranges or limits. There has been no consensus around the suitability of these values, as some centres consider the cited values for characteristics of sperm concentration, morphology and motility too high, whereas others consider them too low. If too high, a high percentage of fertile men would be classified as subnormal, especially when morphology, sperm concentration or motility is considered (Barratt et al., 1988; Chia et al., 1998; Nallella et al., 2006; Pasqualotto et al., 2006; Gao et al., 2007, 2008).
Healthy men may also be investigated for infertility, or inappropriately treated by Assisted Reproduction Technologies, as a result of their lower semen quality if reference limits are too high (Bostofte et al.,1983; Lemcke et al., 1997). On the other hand, a sperm concentration of 20 _ 106/ml, the ‘normal’ or ‘reference‘ value cited by WHO (1987, 1992, 1999), has been considered too low for a lower reference limit because the probability of pregnancy is essentially linear with sperm concentrations up to 40–50 _ 106/ml (Bonde et al., 1998; Slama et al.,2002). Conversely, sperm concentrations above this value are repeatedly observed in infertile patients (Nallella et al., 2006). There may be no upper limit of any semen characteristics since pregnancy rates increase with superior sperm morphology and motility (Garrett et al., 2003). The then-current normal morphology value of WHO (1987) was considered inadequate by Check et al. (1992) as it did not distinguish between fertile and infertile men whose partners were healthy. With uncertain reference values, over- or underdiagnosis may result. Although much of the investigation conducted to date has considered the WHO ‘normal’ or ‘reference’ values as cut-off limits separating fertile from infertile populations, doubts have been raised about the validity of this approach (Bartoov et al., 1993; Barratt et al., 1995).This article considers which men are most suitable for providing a reference population, presents data from such a population, mentions the possible limitations of the results obtained and discusses how the reference intervals could be interpreted as useful reference limits. The present analysis benefits from the availability and incorporation of multi country data from recent fathers with known time-to-pregnancy (TTP). The development and application of clear reference ranges should help reduce the incidence of misdiagnosis of fertility problems and improve clinical care.
Individuals considered suitable for providing reference semen values have included unselected populations, that is, men of unproven fertility (Irvine et al., 1996; Paulsen et al., 1996; Lemcke et al., 1997; Junqing et al. 2002); men from couples presenting with infertility (MacLeod and Wang, 1979; Bostofte et al., 1983; Berling and Wolner-Hanssen,1997; Andolz et al., 1999); candidates for semen donation, some proven fertile (Leto and Frensilli, 1981; Auger et al., 1995; Bujan et al., 1996; Van Waeleghem et al., 1996) and men presenting for vasectomy (Sultan Sheriff, 1983; Fisch et al., 1996). Whereas the first group may be considered drawn from the general population, semen donors may be, and vasectomy candidates most probably are, of proven fertility, although paternity may not have been recent relative to provision of the semen sample analysed. The majority of men have indefinable fertility status at any one moment: therefore a reference range comprising recently fertile men is defined by men whose semen variables may not reflect those of the general population. This is unusual among clinical laboratory tests and clearly presents a major challenge in defining a valid population reference range for human semen. The present study examined semen quality in groups of men from the general population (having unknown fertility status) as well as fathers. For the investigation of male factor infertility, the most relevant reference group is that of proven fertile men, since for valid comparisons of patient data with the reference values, the patient should sufficiently resemble the reference individuals in all respects other than those under investigation (PetitClerc and Solberg,1987;Solberg, 1987), in this case fertility. The selection criteria determining which individuals are included in the reference population would ideally include proof of paternity, but this is rarely requested or obtained.
Where semen samples are sought from fertile men, approaching the pregnant woman is likely to lead to the identiﬁcation of the true biological father; but whether he provides a sample may depend on his cultural and social background, as well as his doubts about pater-nity, biasing the study population unpredictably. Several prospective cross-sectional studies have established baseline values of human semen quality from standardized methodology in relation to fertility (Zinaman et al., 2000; Auger et al., 2001; Jørgensen et al., 2001; Swan, 2003; Eustache et al., 2004; Slama et al., 2004; Haugen et al., 2006; Iwamoto et al., 2006; Pal et al., 2006; Stewart et al., 2009). To avoid the collection bias associated with selecting fertile men, obtaining whole population data has been suggested as ideal. Although theoretically attractive, this is practically unachievable owing to the potentially embarrassing or personal nature of reproductive studies per se (Handelsman et al., 1985), the attitudes of those seeking care (Tielemans et al., 2002) and self-selection of those who are willing to participate (Handelsman, 1997).
The increasing acceptance of WHO standard methodology for semen analysis by laboratories performing clinical studies worldwide means that reference distributions can be generated from a combined analysis of these data. This article presents semen characteristics of, and provides reference intervals and limits generated from, a popu-lation of men who had fathered a child within 1 year of trying to induce a pregnancy. The 95% reference intervals for a range of semen variables and the lower (2.5th centile and 5th centile) reference limits, have been generated, in line with clinical chemistry standards. Data from populations of fathers with unknown TTP and men with unknown fertility status are also presented, to indicate that ranges may be different for men with untested fertility examined for other purposes such as male contraception studies, or recruited from the general population. The present analyses were performed on behalf of, and with ﬁnancial and technical support from, WHO; the data are to be included in the forthcoming ﬁfth edition of the ‘WHO lab-oratory manual for the examination and processing of human semen’. to conceive after at least 12 months of unprotected intercourse (Rowe et al., 1993, 2000).
Semen data from three other groups of men were examined for comparison:
(i) ‘unscreened’ men were men from the general population or young healthy men applying to donate samples for trials of hormonal contra-ception. This is a mixed population of men of unknown fertility, assumed to be representative of the general population. Data from 965 semen samples from seven studies in ﬁve countries on three con-tinents were combined and analysed (Table I).
(ii) ‘screened’ men were those whose samples satisﬁed the then-current WHO criteria for normozoospermia. This is a mixed population of men with unknown fertility history, being either volunteers who were screened prior to participation in male contraceptive trials or men attending infertility clinics. Data are presented to reveal any effects of pre-selection of samples and to represent the population that conformed to previous ‘normal’ or ‘reference’ values. A total of 934 data points from four studies in four countries on three con-tinents and from two multinational WHO studies (WHO, 1990, 1996; Table I) were combined and analysed.
(iii) ‘fertile men with unknown TTP’ were those whose partners gave birth prior to the provision of the semen sample, but with no reported TTP. This is a population of fertile men with partnerships of probably all ranges of fecundity: high, normal, moderately or severely impaired. A total of 817 data points from two studies in two countries on two continents and from two multinational WHO studies (WHO, 1990, 1996; Table I) were combined and analysed.