UC San Diego

Rationale & objective

A low ankle-brachial index (ABI) is used to diagnose peripheral artery disease (PAD) but may be normal or elevated in patients with medial arterial calcification and stiff vessels, as is common in chronic kidney disease (CKD). The toe-brachial index (TBI) has been recommended because it is not influenced by medial arterial calcification, but alone the TBI does not capture risk associated with medial arterial calcification. We hypothesized that the difference between ABI and TBI (ABI - TBI) would capture both PAD and medial arterial calcification and thus better identify mortality risk from PAD, particularly in those with CKD.

Study design

Prospective cohort study.

Setting & participants

471 patients with clinical suspicion for PAD referred for vascular testing.

Exposures

ABI, TBI, and ABI - TBI.

Outcome

All-cause mortality.

Analytical approach

Cox proportional hazards models evaluating the association of ABI - TBI with mortality over 7 years.

Results

Mean age was 68 years, 89% were men, 35% had diabetes, 64% had CKD, and mean estimated glomerular filtration rate was 55 mL/min/1.73 m². Median ABI was 0.96 (interquartile range [IQR], 0.73-1.08), median TBI was 0.62 (IQR, 0.46-0.81), and median ABI - TBI was 0.23 (IQR, 0.14-0.39). Higher ABI - TBI values were associated with increased risk in mortality only among participants with ABI values ≥ 0.9 (P = 0.03). Among participants with CKD and ABI values ≥ 0.9, participants with ABI - TBI values higher than the median had greater (HR, 1.79; 95% CI, 1.18-2.72) risk for mortality (P = 0.005). This was attenuated after age adjustment (HR, 1.41; 95% CI, 0.91-2.20) but did not change after further adjustment for confounders.

Limitations

Mainly male cohort derived from a vascular laboratory; lack of limb outcomes and data for albuminuria.

Conclusions

A high ABI - TBI value may be associated with higher risk for mortality in persons with CKD and a normal ABI. Age affects this association, and further studies evaluating ABI - TBI in larger populations are required.