Necrotizing fasciitis is a rapidly progressive inflammatory infection of the fascia, with secondary necrosis of the subcutaneous tissues. The spectrum of presentation is wide, ranging from a benign-appearing rash in a well person to obvious skin necrosis with hemodynamic instability, multi-organ failure, and death. Patients who present early in this spectrum of disease are difficult to diagnose, with an initial misdiagnosis rate of 71.4 percent.1 However, initiation of treatment in these early stages gives patients the greatest chance of survival in this otherwise deadly disease. In this early phase, necrotizing fasciitis can be mistaken for simple cellulitis, and while the skin may appear benign, it is often the tip of the iceberg to what lies beneath.
The diagnostic difficulty also lies in the fact that there are no lab test results or even imaging that can definitively rule out necrotizing fasciitis. In fact, the diagnosis is a clinical one that can only be confirmed with surgical exploration. Therefore, it is imperative that if you have anything more than the slightest suspicion based on your clinical exam, you consider early consultation with a surgeon for definitive diagnosis and surgical debridement as well as start empiric antibiotics. Lab findings that are suggestive but not diagnostic of necrotizing fasciitis include coagulopathy, hypoalbuminemia, thrombocytopenia, lactic acidosis, creatine phosphokinase elevation, and C-reactive protein (CRP) elevation, which all tend to occur in later stages of disease.1
While clinical decision tools, such as the Laboratory Risk Indicator for Necrotizing Fasciitis (LRINEC) score that includes CRP, white blood cell count, hemoglobin, sodium, creatinine, and glucose, might help raise your suspicion for necrotizing fasciitis, validation studies showed that a LRINEC cutoff of six points only had a negative predictive value of 92.5 percent.2,3 While these lab findings and imaging findings of subcutaneous air and fascial thickening on X-ray, CT, and MRI can help support the diagnosis, they should not delay definitive treatment in the operating room in clinically obvious cases and should never override clinical judgment.
Findings on point-of-care ultrasound, which has the advantage of speed over other imaging modalities, may help support the diagnosis but again cannot rule it out.4 The reason that the absence of subcutaneous air on imaging cannot rule out the diagnosis is that one of the two types of necrotizing fasciitis is caused by non–gas-producing bacteria. In fact, imaging findings are often similar to those of cellulitis, with increased soft-tissue thickness and opacity.5 Gas in the soft tissues is seen in only a minority of cases, but if you do see it, your suspicion for necrotizing fasciitis should be significantly raised.