This analysis sought to develop a tool for early differentiation between pediatric necrotizing fasciitis (NF) of the extremity and more benign infection. All diagnoses of extremity NF, cellulitis, and abscess from a single institution from 2009 to 2015 were included. Vitals and laboratory values were compared between cohorts using analysis of variance and decision tree analysis. NF was associated with thrombocytopenia, hyperthermia, tachycardia, and tachypnea. Decision tree analysis identified C-reactive protein more than 7.0 to be 100% sensitive and 95.7% specific for differentiating NF from cellulitis or abscess. The results offer quantitative support for clinical suspicion of NF in the evaluation of pediatric extremity infection.