title: Embryo cryopreservation: proposal for a new indicator of efficiency.
abstract: OBJECTIVE
  To determine whether "pregnancy rate (PR) per thawed embryo," accounting for embryo losses, is more relevant than "PR per ET," generally used in the literature for expressing embryo cryopreservation results.
  
  
  METHODS
  Systematic review.
  
  
  METHODS
  University teaching hospital.
  
  
  METHODS
  None.
  
  
  METHODS
  Analysis of reports from January 1983 to February 2009 involving thawing of human cryopreserved embryos. Of the 1,275 articles that were initially identified, 102 met the inclusion criteria.
  
  
  METHODS
  Comparisons between PR per ET and PR per thawed embryo, contribution of embryo loss to PR.
  
  
  RESULTS
  Overall PR, expressed per ET and per thawed embryo, was compared according to the different freezing speeds and cryoprotectants used. Statistical analysis revealed significant differences for four comparisons with one approach to expressing results, not identified with the other. All other statistical analyses gave similar results whichever way the results were expressed. The rate of thawed transferred embryos, a measure accounting for embryo losses, is a good prognostic factor for PR.
  
  
  CONCLUSIONS
  Statistical analysis accounting for embryo losses can give results different from those published in the literature. Consequently, the clinical PR per thawed embryo may be a more informative measure for accurate analysis of practices.
authors.affiliation: Reproductive Biology Unit, University Hospital, Hôpital Pitié-Salpêtrière, Paris, France. marieprades@yahoo.fr
authors.forename: Marie
authors.initials: M
authors.lastname: Prades
authors.forename: Jean-Louis
authors.initials: JL
authors.lastname: Golmard
authors.forename: Benoit
authors.initials: B
authors.lastname: Schubert
authors.forename: Catherine
authors.initials: C
authors.lastname: Poirot
mesh_terms: D002970:Cleavage Stage, Ovum / Q000502:physiology; D015925:Cryopreservation / Q000379:methods* / Q000592:standards; D004526:Efficiency; D004624:Embryo Transfer / Q000706:statistics & numerical data; D004622:Embryo, Mammalian; D005260:Female; D006801:Humans; D008954:Models, Biological; D011247:Pregnancy; D018873:Pregnancy Rate; D019984:Quality Indicators, Health Care; D012107:Research Design