The sensitivity of neutrophil CD64 expression as a biomarker of bacterial infection is low in critically ill patients

CD64 expression on the surface of neutrophils has recently been proposed as an early marker of bacterial infection. The goal of this study was to determine whether the CD64 index allows differentiation of bacterial sepsis from viral and fungal sepsis and other inflammatory states in a critical-care setting. This was an observational prospective study conducted in a medical ICU of a university hospital. All patients admitted between September 2009 and March 2010 with at least two criteria for systemic inflammatory response syndrome (SIRS) were eligible for inclusion. Upon admission, hematological exams were conducted by flow cytometry, allowing quantification of CD64 expression (Leuko64 (TM) kit, Trillium Diagnostics LLC, USA). ROC curve analysis was performed to evaluate the utility of the CD64 index in the diagnosis of bacterial infection. Patients with suspected infection were excluded when infection could not be microbiologically confirmed. Our study included 293 patients with a SAPS II score of 45 (31-59). Bacterial infection was found in 148 patients and SIRS or non-bacterial infection was documented in 145 patients. A CD64 index greater than 2.2 predicted bacterial infection with a sensitivity and specificity of 63% (55-71%) and 89% (83-94%), respectively. The area under the ROC curve was 0.8 (0.75-0.84). Positive and negative likelihood ratios were 5.7 (5.0-6.5) and 0.4 (0.3-0.7), respectively. The CD64 index is specific for bacterial infection among ICU patients. As a result of its weak sensitivity, the CD64 index may not be practically recommended, but it may be useful in combination with a more sensitive biological marker.