title: Comparison of the risk factors effects between two populations: two alternative approaches illustrated by the analysis of first and second kidney transplant recipients.
abstract: BACKGROUND
  Whereas the prognosis of second kidney transplant recipients (STR) compared to the first ones has been frequently analyzed, no study has addressed the issue of comparing the risk factor effects on graft failure between both groups.
  
  
  METHODS
  Here, we propose two alternative strategies to study the heterogeneity of risk factors between two groups of patients: (i) a multiplicative-regression model for relative survival (MRS) and (ii) a stratified Cox model (SCM) specifying the graft rank as strata and assuming subvectors of the explicatives variables. These developments were motivated by the analysis of factors associated with time to graft failure (return-to-dialysis or patient death) in second kidney transplant recipients (STR) compared to the first ones. Estimation of the parameters was based on partial likelihood maximization. Monte-Carlo simulations associated with bootstrap re-sampling was performed to calculate the standard deviations for the MRS.
  
  
  RESULTS
  We demonstrate, for the first time in renal transplantation, that: (i) male donor gender is a specific risk factor for STR, (ii) the adverse effect of recipient age is enhanced for STR and (iii) the graft failure risk related to donor age is attenuated for STR.
  
  
  CONCLUSIONS
  While the traditional Cox model did not provide original results based on the renal transplantation literature, the proposed relative and stratified models revealed new findings that are useful for clinicians. These methodologies may be of interest in other medical fields when the principal objective is the comparison of risk factors between two populations.
authors.affiliation: Department of Biostatistics EA 4275, Clinical Research and Subjective Measures in Health Sciences, University of Nantes, 1 rue Gaston Veil, Nantes 44035, France.
authors.forename: Katy
authors.initials: K
authors.lastname: Trébern-Launay
authors.forename: Magali
authors.initials: M
authors.lastname: Giral
authors.forename: Jacques
authors.initials: J
authors.lastname: Dantal
authors.forename: Yohann
authors.initials: Y
authors.lastname: Foucher
mesh_terms: D000367:Age Factors; D005260:Female; D006085:Graft Survival; D006801:Humans; D016030:Kidney Transplantation; D008297:Male; D008875:Middle Aged; D008954:Models, Biological; D016016:Proportional Hazards Models; D012044:Regression Analysis; D012086:Reoperation; D012307:Risk Factors; D012737:Sex Factors; D013997:Time Factors
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