Preoperative BUN-to-Albumin Ratio Is Independently Associated with Major Reamputation After Distal Amputation in Diabetic Foot: A Retrospective Cohort Study

Background: Major level escalation following distal amputation for diabetic foot—defined as subsequent below-knee amputation (BKA)—represents a clinically meaningful endpoint with substantial functional implications. Identifying reliable and readily available preoperative biomarkers capable of predicting major level escalation remains a clinical challenge. This study aimed to evaluate the independent prognostic value of the C-reactive protein-to-albumin ratio (CAR) and blood urea nitrogen-to-albumin ratio (BAR) in predicting postoperative major level escalation. Methods: We retrospectively analyzed 151 consecutive patients who underwent distal lower extremity amputation for diabetic foot between January 2020 and October 2025. The primary outcome was ipsilateral below-knee amputation within the follow-up period. Preoperative CAR and BAR were calculated using laboratory parameters obtained within 24 h prior to surgery. Given the shared albumin component, two separate multivariable logistic regression models were constructed to evaluate independent associations, adjusting for age, peripheral arterial disease (PAD), and index amputation level. Results: During follow-up, 46 patients (30.5%) required major level escalation (BKA). Both CAR and BAR were significantly higher in patients who developed BKA (p < 0.001 and p = 0.006, respectively). In receiver operating characteristic (ROC) analysis, CAR demonstrated acceptable discriminative ability (AUC = 0.745; 95% CI 0.653–0.827), whereas BAR showed modest performance (AUC = 0.640; 95% CI 0.536–0.738). The optimal cutoff values were 1.50 for CAR (sensitivity 76.1%, specificity 61.9%) and 0.61 for BAR (sensitivity 73.9%, specificity 44.8%), although these thresholds were considered exploratory. In the primary multivariable analysis, both CAR (OR 1.16; 95% CI 1.02–1.32; p = 0.024) and BAR (OR 4.02; 95% CI 1.85–8.73; p = 0.005) were independently associated with major level escalation. In sensitivity analyses, BAR retained independent significance, whereas CAR did not (p = 0.100). Conclusions: Preoperative BAR demonstrated robust independent association with major level escalation across both primary and sensitivity analyses, while CAR showed association in the primary model only. These composite biomarkers may provide hypothesis-generating insights into systemic risk profiling in diabetic foot surgery, pending external validation.