ABSTRACT
INTRODUCTION: The national abdominal aortic aneurysm (AAA) screening programme measures internal wall diameter; however, current UK intervention criteria use external wall diameter. Our aim was to determine the clinical significance of the difference between these two measurements. PATIENTS AND METHODS: Fifty patients on an aneurysm surveillance programme were ultrasound scanned by two experienced vascular scientists, blinded to the other's results. Maximum anteroposterior internal wall and maximum anteroposterior external wall diameters were measured. RESULTS: The median difference between internal and external diameter was 6 mm (IQR 6-7) for scientist 1 and 7 mm (IQR 5-8) for scientist 2. This was statistically significant (P < 0.0002). External wall diameter displayed less interobserver variability (3 mm vs 6 mm). CONCLUSIONS: Screening measurements underestimate aneurysm size by 6 mm and display greater variability in comparison to external wall measurements. These findings should be understood to prevent a delay in the detection and treatment of AAAs.