Reliability of radiologic evaluation of abdominal aortic calcification using the 24-point scale

Calcification of the abdominal aorta is associated with increased cardiovascular morbidity, so a reliable method to quantify it is clinically transcendent. The 24-point scale (AAC-24) is the standard method for assessing abdominal aortic calcification on lateral plain films of the lumbar spine. The aim of this study was to determine the intraobserver and interobserver agreements for the AAC-24, taking into account the heterogeneity of the distribution of the calcifications in the design of the statistical analysis.

We analyzed the intraobserver agreement (in plain films from 81 patients, with a four-year separation between observations) and the interobserver agreement (in plain films from 100 patients, with three observers), using both intraclass correlation and Bland-Altman plots.

The intraobserver intraclass correlation coefficient was 0.93 (95% confidence interval [CI95%]: 0.6-0.9), and the interobserver intraclass correlation coefficient was 0.91 (CI95%: 0.8-0.9) with an increase in the coefficient in the tercile with the greatest discrepancy. The difference in means ranged from 0.3 to 1.2 points, and the distance between the limits of agreement ranged from 4.7 to 9.4 points. These differences increased significantly as the calcification progressed.

Using the AAC-24 on lateral plain films of the lumbar spine is a reliable and reproducible method of assessing calcification of the abdominal aorta; both intraobserver and interobserver agreement are higher during the initial phases of calcification.