Quantification of neutrophil and monocyte CD64 expression: a predictive biomarker for active tuberculosis

SETTING: QuantiFERON TB assay (QFT) is used to screen tuberculosis (TB) infection, but it cannot distinguish active TB from latent TB infection (LTBI). OBJECTIVE: To evaluate the quantitative expression of the high-affinity FCgamma receptor I (CD64) on neutrophils (NE) and monocytes (MO) in peripheral blood using flow cytometry, measured in antibody binding capacity (ABC) units as a predictive biomarker of TB. DESIGN: Fifty-two patients were enrolled (45 QFT-positive and 7 QFT-indeterminate). Cultures and molecular analyses were performed. RESULTS: Of the 45 QFT-positive patients, 29 were culture-positive (active TB) and 16 were negative (LTBI). The median NE CD64 ABC and MO CD64 ABC expression was significantly higher ( P < 0.001) in culture-positive patients. The NE CD64 and MO CD64 area under the receiver operating characteristic curve values were respectively 0.948 (95%CI 0.838–0.992) and 0.989 (0.901–1.000). By setting the cut-off NE CD64 value at >2400 ABC or MO CD64 value >25 800 the assay sensitivity increased to 95.5% with 100% specificity and 100% positive predictive value. In the QFT-indeterminate group, five culture-positive cases had NE CD64 >2400 ABC or MO CD64 value >25 800; two culture-negative cases had lower values. CONCLUSION: The CD64 quantitative expression on peripheral blood cells may be used as a predictive biomarker for active TB.