Purpose. Common carotid artery intimal medial thickness (IMT) is a useful screening test for the detection of clinically important atherosclerosis. However, IMT measurement is expensive and not readily available. The ankle-brachial index (ABI) is a cheap and readily available test which is clinically proven for the investigation of suspected peripheral arterial disease (PAD). The use of the ABI as a screening test for atherosclerosis is not well established. The purpose of our investigation was to determine the usefulness of the ABI to detect clinically important atherosclerosis as determined by common carotid artery IMT measurement.
Methods. 213 patients referred for cardiac symptoms were then referred for carotid duplex imaging and peripheral physiologic leg testing. Patients referred specifically for the investigation of suspected PAD were excluded. An ABI of 0.99mm were considered as abnormal. An ABI of >0.94 and an IMT of <1.00mm were considered as normal. All data were entered into CARDIOfile, a cardiology database for later retrieval and analysis. All statistics were based on the common carotid IMT as being the gold standard for the presence or absence of atherosclerosis. Calculations were made for sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV) and accuracy.
Results. See table 1.
Conclusions. The ABI has a low sensitivity (25.4%), low negative predictive value (23.2%) and low overall accuracy (39.0%) when compared to the common carotid IMT for the detection of clinically important atherosclerosis. The specificity of the ABI is high (97.5%) as is the positive predictive value (97.8%). Clinically, the ABI is really only useful when abnormal. A normal ABI should not dissuade one for looking for evidence of atherosclerosis in other areas. The ABI is not a good screening test for atherosclerosis, and we believe that carotid IMT is a far more useful investigative tool.