Classification of carotid stenosis by millimeter CT angiography measures: effects of prevalence and gender

BACKGROUND AND PURPOSE: Previous studies quantifying moderate and severe carotid stenosis by direct millimeter measures on CT angiography (CTA) did not consider how prevalence and gender may influence classification cutoff values.MATERIALS AND METHODS: Three hundred nineteen carotid arteries were evaluated in consecutive patients with known or suspected carotid artery disease. Millimeter measures were obtained of the stenotic carotid bulb lumen and distal internal carotid artery (ICA). Interclass correlation coefficients (ICC) defined interobserver and intraobserver agreement. North American Symptomatic Carotid Endarterectomy Trial (NASCET)-style percent stenosis ratios were calculated per carotid artery and used in linear regression and receiver operating characteristic (ROC) curve analysis to define equivalent millimeter quantification and classification values. Likelihood ratios and prevalence-specific positive/negative predictive values (PPV/NPV) were calculated to determine the most appropriate millimeter cutoff values to classify stenosis.RESULTS: Interobserver agreement was excellent for stenosis measures (0.90) and good for distal ICA measures (0.79). Gender-specific regression curves and ROC curves indicated that millimeter stenosis is an excellent tool to quantify and classify carotid stenosis. Assuming a 10% prevalence of severe stenosis, we found that the cutoff value maximizing NPV and PPV was 1.1 mm for both genders (female: PPV = 86.2, NPV = 97.7; male: PPV = 83.2, NPV = 95.9). Assuming a 40% prevalence of moderate stenosis, we found that the cutoff values differed between genders: female = 2.0 mm (PPV = 91.3, NPV = 91.5), male = 2.1 mm (PPV = 91.6, NPV = 92.4). Specific millimeter cutoffs will vary depending upon the clinical scenario, prevalence, and gender.CONCLUSIONS: Direct millimeter stenosis measures are an excellent tool to classify moderate and severe carotid artery stenosis. Millimeter classification cutoff values that best approximate NASCET classifications vary depending on prevalence and gender.

There is a linear relationship between direct millimeter carotid stenosis measures on CT angiography (CTA) and derived percent stenosis as defined by the North American Symptomatic Carotid Endarterectomy Trial (NASCET).^1–3 This linear relationship allows prediction of NASCET-style percent stenosis from a single direct millimeter measure of stenosis. Quantification of carotid stenosis based on a direct stenosis measure is easy, fast, and reliable.¹ In addition to eliminating the need for ratio calculations, a direct stenosis measure eliminates the variability of NASCET-style ratios due to differences in distal ICA size within and among patients.Beyond quantification of stenosis, the NASCET ratio has been used to categorize carotid stenosis as moderate (≥50%–69%) and severe (≥70%). Because millimeter stenosis measures can predict NASCET-style ratios, it could be implied that specific millimeter values may similarly classify carotid artery disease. Prior studies have reported such millimeter classification thresholds, along with their respective sensitivity and specificity values.¹ However, the sensitivity and specificity values of these millimeter stenosis classifications were <100%. The implication of this difference is that the previously defined millimeter stenosis thresholds are misclassifying some cases of carotid disease.To decrease the degree of misclassifications from the millimeter stenosis measures, classification threshold values should maximize the positive and negative predictive values (PPV, NPV) of the test. A PPV (the probability that the results in a patient with a positive test result are truly positive) of the test depends not only on the sensitivity and specificity of the test but also on the prevalence of the “condition” within the population being studied. The “condition” in this case is the prevalence of moderate and severe stenosis, as defined by NASCET ratios. The potential impact that gender-specific prevalence could have on the accuracy of millimeter stenosis categorization, in substitution for calculating a NASCET ratio, is an important feature to consider before implementing a change in practice.