Classifying Coexisting Diabetic Foot Infections

Patients may have coexisting foot infections of different types. This raises the question of how they should be classified. We reasoned that the most severe of the infections would determine the strength of association with clinical outcomes, such as length of hospital stay, amputation rate, transition to long-term care, and mortality. Therefore, we ranked the infections in a presumptive order of severity and assigned the infection to the most severe category for which they had an ICD-9-CM code. Our presumptive order was Gangrene > Osteomyelitis > Foot ulcer > Cellulitis/abscess of foot > Cellulitis/abscess of toe > Paronychia. This was based on clinical judgment, but corresponds in part to Wagner's classification system for diabetic foot ulcers, which ranks ulcers with gangrene > ulcers with osteomyelitis, > ulcers alone[5 (link),6 (link)]. To improve the homogeneity of the groups, we eliminated from each those patients who had only moderately specific codes for more severe types of infection. Figure 1 shows the flow of patients through this process, the numbers that were classified into each group, and the types of infections among those that remained unclassified.
We assumed that patients were under treatment for diabetic foot infection while in the hospital if, during that hospitalization, they were assigned an ICD-9-CM code for any of the foregoing types of diabetic foot infection.