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Neutrophil CD64 Expression: A Sensitive Diagnostic Marker for Late-Onset Nosocomial Infection in Very Low Birthweight Infants

This study aims to evaluate the diagnostic utilities of four leukocyte surface antigens—two lymphocyte antigens (CD25 and CD45RO) and two neutrophil antigens (CD11b and CD64)—for identification of late-onset nosocomial bacterial infection in preterm, very low birthweight infants, and to define the optimal cutoff value for each marker so that it may act as a reference with which future studies can be compared. Very low birthweight infants in whom infection was suspected when they were >72 h of age were eligible for the study. A full sepsis screen was performed in each episode. IL-6, C-reactive protein, and leukocyte surface antigens (CD25, CD45RO, CD11b, and CD64) were measured at 0 (at the time of sepsis evaluation), 24, and 48 h by standard biochemical methods and quantitative flow cytometric analysis. The diagnostic utilities including sensitivity, specificity, and positive and negative predictive values of each marker and combination of markers for predicting late-onset neonatal infection were determined. One hundred twenty-seven episodes of suspected clinical sepsis were investigated in 80 infants. Thirty-seven episodes were proven infection. The calculated optimal cutoff values for CD25, CD45RO, CD11b, and CD64 were 3,100, 2,900, 10,450, and 4,000 phycoerythrin-molecules bound per cell, respectively. An interim analysis of data after 68 episodes suggested that CD25 and CD45RO were poor predictors of neonatal infection with sensitivity or specificity <75% during a single measurement. Thus, these two markers were excluded from further investigation. In the final analysis, CD64 has the highest sensitivity (95–97%) and negative predictive value (97–99%) at 0 and 24 h after the onset. The addition of IL-6 or C-reactive protein (0 h) to CD64 (24 h) further enhanced the sensitivity and negative predictive value to 100%, and has the specificity and positive predictive value exceeding 88% and 80%, respectively. Neutrophil CD64 expression is a very sensitive marker for diagnosing late-onset nosocomial infection in very low birthweight infants. If further validated, the use of CD64 as an infection marker should allow early discontinuation of antibiotic treatment at 24 h without waiting for the definitive microbiologic culture results. The quantitative flow cytometric analysis applied in this study could be developed into a routine clinical test with high comparability and reproducibility across different laboratories.