Evaluating the Diagnostic Validity of the Ankle-Brachial Index in Diabetic Populations: A Methodological Deconstruction of Pseudo-normality and Its Impact on Health Professional Decision-Making Using a Target Trial Emulation Framework

Abstract

The Ankle-Brachial Index (ABI) is universally mandated for diabetic foot ulcer (DFU) risk stratification, yet its prognostic validity for tissue repair remains structurally inconsistent. To evaluate this diagnostic paradox, we implemented a target trial emulation framework in a dual-campus prospective cohort of 528 patients, utilizing doubly robust estimation and restricted cubic splines to characterize the non-linear relationship between continuous ABI and 20-week healing outcomes. The analysis revealed a profound hemodynamic divergence. The ABI maintained log-linear validity as a predictor against major amputation (OR 0.77 per 0.1-unit increase). In contrast, wound healing followed an inverted U-shaped trajectory. Healing efficacy reached a structural peak at an ABI of approximately 0.90, yielding a 27.9% predicted area reduction. Beyond this threshold, the incremental prognostic benefit of the metric plateaued and eventually collapsed. Sensitivity analyses confirmed that this loss of fidelity is a diagnostic artifact driven by extreme medial arterial calcification (≥1.30) rather than physiological perfusion loss. These findings demonstrate that reliance on categorical ABI thresholds induces a systemic misclassification of microvascular risk. Consequently, clinical guidelines should transition toward continuous hemodynamic profiling and multi-modal assessments to identify occult ischemia in pseudonormalized populations, effectively decoupling diagnostic strategies for limb salvage from tissue repair.