Predictors of Severity in Diabetic Foot Infections

Background: Diabetic foot infections (DFIs) constitute a large burden of the morbidity of diabetes, with more than 70,000 lower-extremity amputations (LEA) performed annually in the United States. A necrotizing infection signifies the most severe form of infection and is a key factor in the decision to proceed to LEA for source control. Key clinical and laboratory variables can assist in the identification of necrotizing infections; however, the effect of diabetes on these variables is unknown. Given the increased level and complexity of metabolic derangements in diabetic patients, we sought to examine characteristics predictive of necrotizing infection in patients with DFI who underwent LEA. Methods: We performed a single-institutional retrospective analysis of diabetic patients who underwent a LEA for DFIs over an 18-month period. Patients with necrotizing infection on final pathology were compared with patients without this pathologic finding. Multivariate analysis was performed to identify independent predictors of necrotizing infection. Results: Of 183 patients, 57 (31%) had evidence of necrotizing infections. Factors associated with necrotizing infection on univariate analysis were the presence of bullae (11% vs. 2%; odds ratio [OR] = 4.8, P = 0.03), a higher mean admission white blood cell count (WBC; 15 vs. 12, P = 0.002), a lower mean absolute sodium (132 vs. 134, P = 0.01), a higher hemoglobin A1C (11.3 vs. 10.3, P = 0.05), hyperglycemia (289 vs. 248, P = 0.04), elevated C-reactive protein (20 vs. 11, P = 0.02), and the presence of Pseudomonas aeruginosa on final tissue culture (12.3 vs. 1.6, P = 0.004). These patients were taken to surgery more rapidly (22.5 vs. 31 hr, P = 0.04), and they had a longer postoperative stay (7 vs. 4 days, P = 0.02). On multivariate analysis, an elevated WBC was predictive of necrotizing infection (OR = 1.1, P = 0.01), whereas alcohol use was found to be protective (OR = 0.3, P = 0.04). Conclusions: Clinical and laboratory variables known to be associated with necrotizing infections among the general population appear to be predictive of disease severity among patients undergoing amputation for DFIs. Identification of these abnormalities preoperatively may allow for improved operative planning, shared decision making, and resource management. Prospective validation of these findings is potentially warranted.