Neutrophil CD64 expression: distinguishing acute inflammatory autoimmune disease from systemic infections.
Common bacterial and opportunistic infections are a major cause of mortality in patients who are immunosuppressed owing to treatment with corticosteroids or cytotoxic drugs. Common laboratory tests for infection lack sensitivity and specificity. One of the new generation of tests to detect early systemic infections measures the up regulation of an Fc receptor (Fcgamma R1, or CD64) on neutrophils. The Fc receptors on white blood cells are very important for effective phagocytosis of bacteria and are up regulated during an infection. To measure the clinical usefulness of quantitative CD64 measurements to differentiate between systemic infection and active autoimmune inflammation in an ongoing study. Patients with systemic infection (n=27), active autoimmune inflammatory disease (n=44), vasculitis (n=5), and controls (n=20) were studied for neutrophil CD64 expression using monoclonal antibodies and flow cytometry. The median (interquartile range (IQR)) CD64 expression in patients with active inflammatory disease and systemic infection was 907.5 (586-1550) and 3647 (2380-6642), respectively (p<0.0001). The median (IQR) CD64 expression in control patients (osteoarthritis and fibromyalgia) was 505 (359-599). The sensitivity and specificity of CD64 expression on neutrophils to diagnose systemic infection (using a cut off value of 2000) was 85% and 91%, respectively. These results indicate that quantitative measurement of CD64 can distinguish between systemic infection and the flare of autoimmune diseases.