title: Categorizing BMI may lead to biased results in studies investigating in-hospital mortality after isolated CABG.
abstract: OBJECTIVE
  To investigate how categorizing body mass index (BMI) into weight classes can impact the assessment of the relationship between BMI and in-hospital mortality after coronary artery bypass graft (CABG) surgery.
  
  
  METHODS
  BMI-mortality (in-hospital) relationship was assessed in 5,762 patients who underwent isolated CABG at Baylor University Medical Center (Dallas, TX) from January 1, 1997 to November 30, 2003. Different ways of modeling BMI were used to investigate this association in a propensity-adjusted model, controlling for risk factors identified by the Society of Thoracic Surgeons (STS) and other clinical/nonclinical details.
  
  
  RESULTS
  A highly significant (P=0.003) association between BMI (modeled with a restricted cubic spline) and mortality was found. Reduced risk of in-hospital mortality was observed for subjects with BMI in the low-30s as compared with patients with BMI in the mid-20s or over 40 kg/m(2). Results were strongly affected by the way BMI was specified in the multivariable model. Only five of the 10 BMI categorizations considered produced significant results, and these results did not fully determine the effect of BMI on mortality.
  
  
  CONCLUSIONS
  BMI categorization critically impacts study results. Conceivably, findings of other studies investigating BMI and adverse outcomes after CABG may be similarly affected. Investigators should consider smoothing techniques to avoid categorization.
authors.affiliation: Institute for Health Care Research and Improvement, Baylor Health Care System, Dallas, TX 75206, USA. giovanni.filardo@aya.yale.edu
authors.forename: Giovanni
authors.initials: G
authors.lastname: Filardo
authors.forename: Cody
authors.initials: C
authors.lastname: Hamilton
authors.forename: Baron
authors.initials: B
authors.lastname: Hamman
authors.forename: Hon K T
authors.initials: HK
authors.lastname: Ng
authors.forename: Paul
authors.initials: P
authors.lastname: Grayburn
mesh_terms: D015982:Bias; D015992:Body Mass Index; D015331:Cohort Studies; D001026:Coronary Artery Bypass / Q000401:mortality*; D005260:Female; D017052:Hospital Mortality; D006801:Humans; D008297:Male; D008875:Middle Aged; D015233:Models, Statistical; D011183:Postoperative Complications / Q000401:mortality; D012307:Risk Factors; D016896:Treatment Outcome