title: Probabilities of progression of aortic aneurysms: estimates and implications for screening policy.
abstract: BACKGROUND
  Screening for abdominal aortic aneurysm, and intervention with elective repair, can reduce the incidence of aneurysmal rupture by a half. If a screening programme is implemented, it is essential to determine appropriate follow up intervals for rescreening. This paper estimates probabilities of progression growth of aortic diameter to provide evidence for this.
  
  
  METHODS
  Data were taken from 2342 men aged 65-80 screened in the Chichester randomised control trial, who have been followed up for an average of 11 years. Aortic diameter was modelled as a Markov process with four categories: <30 mm (normal), 30-44 mm, 45-54 mm, and > or =55 mm. Estimates of the probabilities of progressing to each higher category were obtained.
  
  
  RESULTS
  The probabilities of progression increased with greater initial aortic diameter. The estimated rates/year were 0.018 (95% confidence interval 0.014 to 0.023), 0.16 (0.12 to 0.20), and 0.49 (0.35 to 0.70) respectively for moving up one category. The probabilities of moving from <30 mm to > or =55 mm were estimated as 1% in 5 years and 12% in 15 years, while the corresponding figures for moving from 45-54 mm to > or =55 mm were 91% and 99%. There were differences in rates of progression according to age, with men over 70 years having rates about three times those of men under 70.
  
  
  CONCLUSIONS
  It seems unnecessary to follow up men with normal aortic diameter as they experience a low probability of reaching criteria for surgery even within 15 years. However, follow up intervals should be progressively shorter for those with greater aortic diameter, especially in those aged over 70. Active follow up, for example every 3 months, is appropriate for men with an aortic diameter of 45-54 mm.
authors.affiliation: MRC Biostatistics Unit, Institute of Public Health, Robinson Way, Cambridge CB2 2SR, UK.
authors.forename: E
authors.initials: E
authors.lastname: Couto
authors.forename: S W
authors.initials: SW
authors.lastname: Duffy
authors.forename: H A
authors.initials: HA
authors.lastname: Ashton
authors.forename: N M
authors.initials: NM
authors.lastname: Walker
authors.forename: J P
authors.initials: JP
authors.lastname: Myles
authors.forename: R A P
authors.initials: RA
authors.lastname: Scott
authors.forename: S G
authors.initials: SG
authors.lastname: Thompson
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D001011:Aorta / Q000473:pathology; D017544:Aortic Aneurysm, Abdominal / Q000175:diagnosis / Q000473:pathology*; D018450:Disease Progression; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D008390:Markov Chains; D011336:Probability