Radiologic assessment of abdominal aortic calcifications, atherosclerotic burden levels and statistical bias affecting the reliability

Abstract

IntroductionAAC-8 and AAC-24 are two widely used scales to evaluate abdominal aortic calcification (AAC) on X-ray images. Levels of ≥3 (AAC-8) and ≥5 points (AAC-24) are of high relevance since they are associated with greater risk of cardiovascular events. Given that it is unknown, our aim was to determine the reliability of both scales at those levels of atherosclerotic burden. MethodsThe sample (93 subjects, 67.3 ± 9.7 years, BMI 28.8 ± 3.8, 57.6% smokers, 64.1% with hypertension) was classified according to quartiles of calcification. Six clinicians evaluated AAC independently with both scales on lateral lumbar spine X-ray images. We analyzed inter-rater agreement with the intraclass correlation coefficient (ICC) and the Bland-Altman scatterplots. ResultsWe assessed 15 pairs of raters. Scores in both scales were significantly correlated with cardiovascular risk (r = 0.31 and r = 0.32; p < 0.005). Agreement was very high in the first quartile and moderate in the rest (p < 0.05). At cut-off points, ICC = 0.70 (95%CI, 0.54–0.86) and ICC = 0.68 (95%CI, 0.60–0.85) with AAC-8 and AAC-24. With the Bland-Altman method, mean of the differences ranged between 0 and 0.4 (AAC-8), and between 0.2 and 1 (AAC-24), while 95% limits of agreement showed values between 2.9 and 4.4 (AAC-8), and between 6 and 11.2 (AAC-24). Analyzing entire scales, ICC = 0.97 (95%CI, 0.97–0.98) and ICC = 0.98 (95%CI, 0.97–0.98) for AAC-8 and AAC-24, respectively. ConclusionBoth scales presented only moderate reliability at levels of atherosclerotic burden. Analyzing quartiles with ICC and the Bland-Altman plot showed concordant results. High global ICC values traditionally reported with both scales are likely biased. Implications for practiceAAC predicts subsequent vascular morbidity and mortality and should implicate evaluation of cardiovascular risk. Optimal visualisation of AAC and its correct assessment are mandatory in order to maximize patient care.