Toe brachial index and not ankle brachial index is a surrogate to CT angiography in the evaluation of peripheral arterial disease in type 2 diabetes

Abstract

Abstract Background Non-invasive clinic-based tools for assessing PAD are not without limitations. Therefore, costly tests like Doppler study, CT angiography and MR angiography must be resorted to. Ankle Brachial Index (ABI), commonly used for assessment of PAD, have high false positivity rates in sclerosed, calcified arteries which render them non-compressible. Toe Brachial Index (TBI) can be an alternative, as digital arteries are relatively unaffected by these changes. Aim To compare the reliability of ABI and TBI in diagnosing PAD using CT angiography (CTA) as the reference and identify the best surrogate to CTA. Methods A total of 350 limbs from 175 adults with T2D were selected. ABI &TBI were measured with an automated vascular Doppler XT 6 ports bilaterally for all subjects. For any subject, the limb with lower ABI and TBI was included for analysis. ABI < 0.9 & TBI < 0.6 were taken as evidence of PAD. CTA showing > 50% narrowing was taken as evidence of PAD. Results 24% of our study subjects had CTA confirmed PAD. ABI has low sensitivity of 38% (95% CI: 0.21–0.52) compared to TBI being 90% (95% CI: 0.66–0.92). The specificity however was similar. ABI < 0.9 was able to detect CTA confirmed PAD, but ABI > 0.9, including the so-called normal ABI (0.9–1.3) was unable to detect PAD. ROC showed ABI at 1.005 has sensitivity 64.71% (95% CI: 0.48–0.79) and specificity 61.7% (95% CI: 0.53–0.69) and TBI at 0.6 has sensitivity 82.35% (95% CI: 0.66–0.92) & specificity 92% (95% CI:0.87–0.96). Utilizing Cohen’s Kappa, the reliability of ABI with respect to CTA showed fair agreement (K = 0.225, p = 0.001), whereas the reliability of TBI with respect to CTA showed substantial agreement (K = 0.759, p < 0.0001) Conclusion ABI < 0.9 detects PAD reliably, but presence of PAD in patients with ABI > 9.0 including the normal of ABI (0.9–1.3) can be confirmed with TBI, which correlated strongly with CTA. TBI is also non-inferior for PAD detection, when ABI < 0.9. TBI and not ABI can be utilized as an apt surrogate to CTA for assessment of PAD in subjects with T2D.