title: Reducing Inter-Laboratory Differences between Semen Analyses Using Z Score and Regression Transformations.
abstract: BACKGROUND
  Standardization of the semen analysis may improve reproducibility. We assessed variability between laboratories in semen analyses and evaluated whether a transformation using Z scores and regression statistics was able to reduce this variability.
  
  
  METHODS
  We performed a retrospective cohort study. We calculated between-laboratory coefficients of variation (CVB) for sperm concentration and for morphology. Subsequently, we standardized the semen analysis results by calculating laboratory specific Z scores, and by using regression. We used analysis of variance for four semen parameters to assess systematic differences between laboratories before and after the transformations, both in the circulation samples and in the samples obtained in the prospective cohort study in the Netherlands between January 2002 and February 2004.
  
  
  RESULTS
  The mean CVBwas 7% for sperm concentration (range 3 to 13%) and 32% for sperm morphology (range 18 to 51%). The differences between the laboratories were statistically significant for all semen parameters (all P<0.001). Standardization using Z scores did not reduce the differences in semen analysis results between the laboratories (all P<0.001).
  
  
  CONCLUSIONS
  There exists large between-laboratory variability for sperm morphology and small, but statistically significant, between-laboratory variation for sperm concentration. Standardization using Z scores does not eliminate between-laboratory variability.
authors.affiliation: Department of Obstetrics and Gynecology, Vrije Universiteit Medical Center, Amsterdam, The Netherlands; Academic Medical Center, Center for Reproductive Medicine, Amsterdam, The Netherlands.
authors.forename: Esther
authors.initials: E
authors.lastname: Leushuis
authors.affiliation: Radboud University Nijmegen Medical Centre, Department of Obstetrics and Gynaecology, Nijmegen, The Netherlands.
authors.forename: Alex
authors.initials: A
authors.lastname: Wetzels
authors.affiliation: Jeroen Bosch Ziekenhuis, Department of Obstetrics and Gynaecology, 's-Hertogenbosch, The Netherlands.
authors.forename: Jan Willem
authors.initials: JW
authors.lastname: van der Steeg
authors.affiliation: University Medical Center Utrecht (UMC Utrecht), Department of Obstetrics and Gynaecology, Utrecht, The Netherlands.
authors.forename: Pieternel
authors.initials: P
authors.lastname: Steures
authors.affiliation: Academic Medical Center, Department of Clinical Epidemiology and Biostatistics, Amsterdam, The Netherlands.
authors.forename: Patrick M M
authors.initials: PM
authors.lastname: Bossuyt
authors.affiliation: Department of Clinical Chemical, St. Antonius Hospital, Nieuwegein, The Netherlands.
authors.forename: Netty
authors.initials: N
authors.lastname: van Trooyen
authors.affiliation: Radboud University Nijmegen Medical Centre, Department of Obstetrics and Gynaecology, Nijmegen, The Netherlands.
authors.forename: Sjoerd
authors.initials: S
authors.lastname: Repping
authors.affiliation: Diagnostic Center SSDZ Reinier De Graaf Group, Department of Clinical Chemistry, Delft, The Netherlands.
authors.forename: Frans A L
authors.initials: FA
authors.lastname: van der Horst
authors.affiliation: Department of Obstetrics and Gynecology, Vrije Universiteit Medical Center, Amsterdam, The Netherlands.
authors.forename: Peter G A Hompes
authors.initials: PG
authors.lastname: Hompes
authors.affiliation: Academic Medical Center, Center for Reproductive Medicine, Amsterdam, The Netherlands.
authors.forename: Ben Willem J
authors.initials: BW
authors.lastname: Mol
authors.affiliation: Academic Medical Center, Center for Reproductive Medicine, Amsterdam, The Netherlands.
authors.forename: Fulco
authors.initials: F
authors.lastname: van der Veen
keywords: Differences; Regression; Semen Analysis; Standardization