INT J TUBERC LUNG DIS 24(2):196–201 Q 2020 The Union http://dx.doi.org/10.5588/ijtld.19.0147 Quantification of neutrophil and monocyte CD64 expression: a predictive biomarker for active tuberculosis A. Gatti, 1 C. Ceriani, 2 M. De Paschale, 2 C. Magnani, 3 M. Villa, 3 P. Vigan ` o, 3 P. Clerici, 2 B. Brando 1 1 Haematology Laboratory and Transfusion Centre, 2 Microbiology Unit, 3 Infectious Diseases Unit, Legnano General Hospital, Legnano, Milan, Italy SUMMARY SETTING: QuantiFERON TB assay (QFT) is used to screen tuberculosis (TB) infection, but it cannot distin- guish active TB from latent TB infection (LTBI). OBJECTIVE: To evaluate the quantitative expression of the high-affinity FCgamma receptor I (CD64) on neutro- phils (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 mo- lecular 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. KEY WORDS: CD64; active TB; latent TB; QuantiFER- ON; flow cytometry ABOUT 23% OF THE WORLD population are estimated to have latent tuberculosis infection (LTBI) with a high risk of developing active tuberculosis (TB) during their lifetime. 1 At present, only the Quanti- FERON w TB assay (QFT; Qiagen, Hilden, Germany) can be used in peripheral blood for LTBI screening. This method, however, cannot distinguish active TB from LTBI and it is not useful for a rapid monitoring of treatment response. 2 Furthermore, a significant proportion of QFT test results are reported as indeterminate, especially in patients with auto- immune diseases or in patients co-infected with the human immunodeficiency virus (HIV). 3,4 Culture- based methods are the current reference gold stan- dard for the diagnosis of active TB, but they require a high degree of laboratory skill and more than 12 weeks to provide results. 1 A recent study shows that a transcriptional increase of several leukocyte genes, including the ones that code for cluster of differentiation 64 (CD64; high- affinity Fcc receptor I), is involved in active TB. 5 This receptor is constitutively expressed by macrophages, monocytes and immature myeloid cells; conversely, quiescent neutrophils (NE) express membrane CD64 at very low density. 6 CD64 is rapidly up-regulated on monocytes (MO) and on NE as a response to the exposure to microbial wall components or to the release of some cytokines (interferon-gamma, tumour necrosis factor alpha, interleukin [IL] 8 and IL12). 7 Therefore, as reported in many studies, the quantita- tive level of CD64 expression is significantly higher in patients with sepsis than in those without systemic infections. 8–10 In addition, some reports have dem- onstrated that circulating NEs from patients with active TB show Mycobacterium tuberculosis-induced activation. 11,12 The aim of the present study was the evaluation of the quantitative CD64 expression on the surface of NE and MO in peripheral blood samples using a highly standardised and calibrated flow cytometric assay as a possible biomarker of active TB. METHODS Patient group From 2010 to 2018, 52 patients were enrolled in this study. Of these, 45 patients (12 females and 33 males) with a median age of 45 years (range 16–85) were Correspondence to: Arianna Gatti, Hematology Laboratory and Transfusion Center, Western Milan Area Hospital Consortium, Legnano General Hospital, 20025 Legnano, Milan, Italy. email: arianna.gatti@asst-ovestmi.it Article submitted 28 May 2019. Final version accepted 6 August 2019.