Getting it Right the First Time: Frozen Sections for Diagnosing Necrotizing Soft Tissue Infections

Background: The aim of this study was to investigate which histopathologic findings are most indicative for necrotizing soft tissue infections (NSTIs) in ambivalent cases. Methods: Patients undergoing surgical exploration for suspected NSTIs with obtainment of incisional biopsies for histopathological assessment were included from January 2013 until August 2019. The frozen sections and formalin-fixed paraffin-embedded (FFPE) samples were retrospectively re-assessed. The primary outcome was the discharge diagnosis. Results: Twenty-seven (69%) biopsies of the 39 included samples were from patients with NSTIs. Microscopic bullae (p = 0.043), severe fascial inflammation (p < 0.001) and fascial necrosis (p < 0.001) were significantly more often present in the NSTI group compared to the non-NSTI group. Muscle edema (n = 5), severe muscle inflammation (n = 5), muscle necrosis (n = 8), thrombosis (n = 10) and vasculitis (n = 5) were most frequently only seen in the NSTI group. In thirteen tissues samples, there were some discrepancies between the severity of findings in the frozen section and the FFPE samples. None of these discrepancies resulted in a different diagnosis or treatment strategy. Conclusion: Microscopic bullae, severe fascial or muscle inflammation, fascial or muscle necrosis, muscle edema, thrombosis and vasculitis upon histopathological evaluation all indicate a high probability of a NSTI. At our institution, diagnosing NSTIs is aided by using intra-operative frozen section as part of triple diagnostics in ambivalent cases. Based on the relation between histopathologic findings and final presence of NSTI, we recommend frozen section for diagnosing NSTIs in ambivalent cases.