title: Impact of body mass index on risk of acute kidney injury and mortality in elderly patients undergoing hip fracture surgery.
abstract: The literature is limited regarding risk factors for acute kidney injury (AKI) and mortality in hip fracture patients, although AKI is common in these patients. While obese patients were at increased risk of AKI, underweight patients with and without AKI had elevated mortality for up to 1 year after hip fracture surgery, compared with normal-weight patients.
  
  
  
  This study aimed to examine risk of postoperative AKI and subsequent mortality, by body mass index (BMI) level, in hip fracture surgery patients aged 65 and over.
  
  
  
  A regional cohort study using medical databases was used. We included all patients who underwent surgery to repair a hip fracture during the years 2005-2011 (n = 13,529) at hospitals in Northern Denmark. We calculated cumulative risk of AKI by BMI level during 5 days postsurgery and subsequent short-term (6-30 days postsurgery) and long-term (31-365 days post-surgery) mortality. We calculated crude and adjusted hazard ratios (aHRs) for AKI and death with 95% confidence intervals (CIs), comparing underweight, overweight, and obese patients with normal-weight patients.
  
  
  
  Risks of AKI within five postoperative days were 11.9, 10.1, 12.5, and 17.9% for normal-weight, underweight, overweight, and obese patients, respectively. Among those who developed AKI, short-term mortality was 14.1% for normal-weight patients compared to 23.1% for underweight (aHR 1.7 (95% CI 1.2-2.4)), 10.7% for overweight (aHR 0.9 (95% CI 0.6-1.1)), and 15.2% for obese (aHR 0.9 (95% CI 0.6-1.4)) patients. Long-term mortality was 24.5% for normal-weight, 43.8% for underweight (aHR 1.6 (95% CI 1.0-2.6)), 20.5% for overweight (aHR 0.8 (95% CI 0.6-1.2)), and 21.4% for obese (aHR 1.1 (95% CI 0.7-1.8) AKI patients. Similar associations between BMI and mortality were observed among patients without postoperative AKI, although the absolute mortality risk estimates by BMI were considerably lower in patients without than in those with AKI.
  
  
  
  Obese patients were at increased risk of AKI compared with normal-weight patients. Among patients with and without postoperative AKI, overweight and obesity were not associated with mortality. Compared to normal-weight patients, underweight patients had elevated mortality for up to 1 year after hip fracture surgery irrespective of the presence of AKI. The absolute mortality risks were higher in all BMI groups with the presence of AKI.
authors.affiliation: Department of Clinical Epidemiology, Aarhus University Hospital, Olof Palmes Alle 43-45, 8200, Aarhus N, Denmark. abp@clin.au.dk.
authors.forename: A B
authors.initials: AB
authors.lastname: Pedersen
authors.affiliation: Department of Clinical Epidemiology, Aarhus University Hospital, Olof Palmes Alle 43-45, 8200, Aarhus N, Denmark.
authors.forename: H
authors.initials: H
authors.lastname: Gammelager
authors.affiliation: Department of Clinical Epidemiology, Aarhus University Hospital, Olof Palmes Alle 43-45, 8200, Aarhus N, Denmark.
authors.forename: J
authors.initials: J
authors.lastname: Kahlert
authors.affiliation: Department of Clinical Epidemiology, Aarhus University Hospital, Olof Palmes Alle 43-45, 8200, Aarhus N, Denmark.
authors.forename: H T
authors.initials: HT
authors.lastname: Sørensen
authors.affiliation: Department of Clinical Epidemiology, Aarhus University Hospital, Olof Palmes Alle 43-45, 8200, Aarhus N, Denmark.
authors.forename: C F
authors.initials: CF
authors.lastname: Christiansen
mesh_terms: D058186:Acute Kidney Injury / Q000209:etiology* / Q000401:mortality; D000368:Aged; D000369:Aged, 80 and over; D015992:Body Mass Index; D016208:Databases, Factual; D003718:Denmark / Q000453:epidemiology; D005260:Female; D005593:Fracture Fixation, Internal / Q000009:adverse effects*; D006620:Hip Fractures / Q000401:mortality / Q000601:surgery*; D006801:Humans; D008297:Male; D009765:Obesity / Q000150:complications* / Q000401:mortality; D016016:Proportional Hazards Models; D012307:Risk Factors; D013851:Thinness / Q000150:complications / Q000401:mortality
keywords: Acute kidney injury; Body mass index; Cohort study; Hip fracture; Mortality