title: Modeling the Growth of Infrarenal Abdominal Aortic Aneurysms.
abstract: BACKGROUND
  Abdominal aortic aneurysm (AAA) growth is a complex process that is incompletely understood. Significant heterogeneity in growth trajectories between patients has led to difficulties in accurately modeling aneurysm growth across cohorts of patients. We set out to compare four models of aneurysm growth commonly used in the literature and confirm which best fits the patient data of our AAA cohort.
  
  
  METHODS
  Patients with AAA were included in the study if they had two or more abdominal ultrasound scans greater than 3 months apart. Patients were censored from analysis once their AAA exceeded 5.5 cm. Four models were applied using the R environment for statistical computing. Growth estimates and goodness of fit (using the Akaike Information Criterion, AIC) were compared, with p-values based on likelihood ratio testing.
  
  
  RESULTS
  Of 510 enrolled patients, 264 met the inclusion criteria, yielding a total of 1861 imaging studies during 932 cumulative years of surveillance. Overall, growth rates were: (1) 0.35 (0.31,0.39) cm/yr in the growth/time calculation, (2) 0.056 (0.042,0.068) cm/yr in the linear regression model, (3) 0.19 (0.17,0.21) cm/yr in the linear multilevel model, and (4) 0.21 (0.18,0.24) cm/yr in the quadratic multilevel model at time 0, slowing to 0.15 (0.12,0.17) cm/yr at 10 years. AIC was lowest in the quadratic multilevel model (1508) compared to other models (P < 0.0001).
  
  
  CONCLUSIONS
  AAA growth was heterogeneous between patients; the nested nature of the data is most appropriately modeled by multilevel modeling techniques.
authors.affiliation: Multidisciplinary Cardiovascular Research Centre, Division of Cardiovascular and Diabetes Research, The Leeds Institute of Genetics, Health and Therapeutics, The University of Leeds, Leeds, United Kingdom; ; The Leeds Vascular Institute, The General Infirmary at Leeds, Leeds, United Kingdom; and.
authors.forename: Marc A
authors.initials: MA
authors.lastname: Bailey
authors.affiliation: The Division of Epidemiology and Biostatistics, The Leeds Institute of Genetics, Health and Therapeutics, The University of Leeds, Leeds, United Kingdom.
authors.forename: Paul D
authors.initials: PD
authors.lastname: Baxter
authors.affiliation: The Division of Epidemiology and Biostatistics, The Leeds Institute of Genetics, Health and Therapeutics, The University of Leeds, Leeds, United Kingdom.
authors.forename: Tao
authors.initials: T
authors.lastname: Jiang
authors.affiliation: Multidisciplinary Cardiovascular Research Centre, Division of Cardiovascular and Diabetes Research, The Leeds Institute of Genetics, Health and Therapeutics, The University of Leeds, Leeds, United Kingdom; ; The Division of Epidemiology and Biostatistics, The Leeds Institute of Genetics, Health and Therapeutics, The University of Leeds, Leeds, United Kingdom.
authors.forename: Aimee M
authors.initials: AM
authors.lastname: Charnell
authors.affiliation: Multidisciplinary Cardiovascular Research Centre, Division of Cardiovascular and Diabetes Research, The Leeds Institute of Genetics, Health and Therapeutics, The University of Leeds, Leeds, United Kingdom; ; The Leeds Vascular Institute, The General Infirmary at Leeds, Leeds, United Kingdom; and.
authors.forename: Kathryn J
authors.initials: KJ
authors.lastname: Griffin
authors.affiliation: Multidisciplinary Cardiovascular Research Centre, Division of Cardiovascular and Diabetes Research, The Leeds Institute of Genetics, Health and Therapeutics, The University of Leeds, Leeds, United Kingdom;
authors.forename: Anne B
authors.initials: AB
authors.lastname: Johnson
authors.affiliation: Multidisciplinary Cardiovascular Research Centre, Division of Cardiovascular and Diabetes Research, The Leeds Institute of Genetics, Health and Therapeutics, The University of Leeds, Leeds, United Kingdom; ; The Leeds Vascular Institute, The General Infirmary at Leeds, Leeds, United Kingdom; and.
authors.forename: Katherine I
authors.initials: KI
authors.lastname: Bridge
authors.affiliation: Multidisciplinary Cardiovascular Research Centre, Division of Cardiovascular and Diabetes Research, The Leeds Institute of Genetics, Health and Therapeutics, The University of Leeds, Leeds, United Kingdom;
authors.forename: Soroush
authors.initials: S
authors.lastname: Sohrabi
authors.affiliation: Multidisciplinary Cardiovascular Research Centre, Division of Cardiovascular and Diabetes Research, The Leeds Institute of Genetics, Health and Therapeutics, The University of Leeds, Leeds, United Kingdom; ; The Leeds Vascular Institute, The General Infirmary at Leeds, Leeds, United Kingdom; and.
authors.forename: D Julian A
authors.initials: DJ
authors.lastname: Scott
keywords: Abdominal aortic aneurysm; Growth rate; Multilevel modeling; Quadratic