title: Single or double semen samples: the dilemma in epidemiological studies on semen quality.
abstract: OBJECTIVE
  Is it necessary to collect more than one semen sample from each individual in epidemiological studies?
  
  
  CONCLUSIONS
  Single semen samples can be used in model construction as long as the model adjusts for major relevant covariates.
  
  
  BACKGROUND
  If a series of semen samples are taken from one individual at different times, significant intra-individual variation may exist.
  
  
  METHODS
  A population-based cross-sectional study was conducted from 1 December 2000 to 20 November 2002. A total of 1010 volunteers, aged between 20 and 60 years old, were recruited.
  
  
  METHODS
  A total of 998 subjects were included for statistical analysis. Of these, 332 men provided single semen samples (group 1) and 666 men provided two samples (group 2). Semen parameters, including semen volume, sperm concentration, total sperm number, progressive motility, vitality, and sperm morphology, were analyzed with standardized methods according to the World Health Organization (WHO) laboratory manual for the examination of human semen and sperm-cervical mucus interaction (3rd edition).
  
  
  RESULTS
  Rapidly progressive motility of semen from group 1 was lower than that of group 2, but other parameters were not significantly different. The differences in mean values between the double samples were small on all semen parameters. The correlations between the two ejaculates were high, but the agreements were poor except for sperm vitality and normal morphology. Model selection and validation analysis supported the premise that the regression model from the first samples was applicable for the second samples. The large sample size with extensive quality control provides robust parameter estimation and promises good applicability for model selection and validation.
  
  
  CONCLUSIONS
  The results are valid for healthy men in China, not for subfertile or infertile men.
  
  
  CONCLUSIONS
  The overall semen quality of a given group of healthy men will not vary significantly in the short term. Single semen samples can be used in model building as long as major relevant covariates are considered. Utilizing multiple semen donations may actually cause the samples to be less representative.
  
  
  BACKGROUND
  This research was supported by a grant from Shanghai Science and Technical Committee (02DJ14053) and a grant from Ministry of Science and Technology, China (9902). All authors have no competing interests.
authors.affiliation: Department of Reproductive Epidemiology and Social Science, National Population and Family Planning Key Laboratory of Contraceptive Drugs and Devices, Shanghai Institute of Planned Parenthood Research, Shanghai 200237, People's Republic of China.
authors.forename: Qian-Xi
authors.initials: QX
authors.lastname: Zhu
authors.affiliation: Department of Reproductive Epidemiology and Social Science, National Population and Family Planning Key Laboratory of Contraceptive Drugs and Devices, Shanghai Institute of Planned Parenthood Research, Shanghai 200237, People's Republic of China.
authors.forename: Er-Sheng
authors.initials: ES
authors.lastname: Gao
authors.affiliation: Women's Health Research Unit, Queen Mary University of London, London, UK.
authors.forename: Neha
authors.initials: N
authors.lastname: Pathak
authors.affiliation: Department of Reproductive Epidemiology and Social Science, National Population and Family Planning Key Laboratory of Contraceptive Drugs and Devices, Shanghai Institute of Planned Parenthood Research, Shanghai 200237, People's Republic of China.
authors.forename: Jun-Qing
authors.initials: JQ
authors.lastname: Wu
authors.affiliation: Department of Reproductive Epidemiology and Social Science, National Population and Family Planning Key Laboratory of Contraceptive Drugs and Devices, Shanghai Institute of Planned Parenthood Research, Shanghai 200237, People's Republic of China zw0822@sina.com.
authors.forename: Wei-Jin
authors.initials: WJ
authors.lastname: Zhou
mesh_terms: D000328:Adult; D003430:Cross-Sectional Studies; D004812:Epidemiologic Methods; D006801:Humans; D008297:Male; D008875:Middle Aged; D008962:Models, Theoretical; D012044:Regression Analysis; D015203:Reproducibility of Results; D055101:Semen Analysis
keywords: epidemiology; intra-individual variation; model selection; semen analysis; sperm motility