There is a clear need for improved indicators of infection or sepsis to increase the sensitivity and specificity of both diagnosis and therapeutic monitoring. One of the effects of inflammatory cytokines on the innate immune response is the rapid up-regulation of CD64 expression on the neutrophil membrane. We and others have hypothesized that the measurement of neutrophil CD64 expression might represent an improved diagnostic indicator of infection and sepsis. In this study we assessed the relative ability of flow cytometric neutrophil CD64 measurements, neutrophil counts, myeloid immaturity differential counts, and flagging on an automated hematology analyzer to correlate with the presence of infection, as determined by a retrospective clinical scoring system of infection or sepsis. A total of 160 blood samples were randomly selected to derive equal proportions of the 3 categories of flags on a Coulter STKS blood counter that indicate the presence of a myeloid left shift. The patients for these samples were scored by retrospective chart review and placed into 4 groups on the basis of likelihood of infection, sepsis, or severe tissue injury. Neutrophil CD64 expression demonstrated a superior sensitivity (94.1%), specificity (84.9%), and positive predictive likelihood ratio (6.24), compared with neutrophil counts (sensitivity, 79.4%; specificity, 46.8%; positive predictive likelihood ratio, 1.49), band counts (sensitivity, 87.5%; specificity, 43.5%; positive predictive likelihood ratio, 1.55), myeloid immaturity fraction (sensitivity, 94.6%; specificity, 84.5%; positive predictive likelihood ratio, 2.12), and flagging on an automated hematology analyzer (sensitivity, 94.1%; specificity, 40.5%; positive predictive likelihood ratio, 1.58). Relative to the other laboratory parameters, the neutrophil CD64 parameter also provided the best separation of the 4 clinical groups. The findings indicate that neutrophil CD64 expression as determined by quantitative flow cytometry is an improved diagnostic indicator of infection/sepsis relative to current laboratory indicators of relative or absolute myeloid cell counts or hematology analyzer flagging algorithms.