Sepsis is a systemic response of the body to an infection, often resulting in the dysfunction of multiple organs and the failure of multiple systems. It remains a syndrome associated with high mortality rates worldwide, being responsible for millions of deaths every year and standing as one of the leading causes of death among hospitalized patients (Singer et al., 2016; Rudd et al., 2020). Sepsis is a condition that requires immediate and adequate treatment, and its rapid identification is crucial to initiating proper treatment and improving clinical outcomes.

To standardize the nomenclature of infectious responses, the Society of Critical Care Medicine (SCCM) and the American College of Chest Physicians have established criteria for the definition of sepsis, severe sepsis, and septic shock (Bone et al., 1992). Recently, new definition criteria for sepsis were established. The Third International Consensus Definitions for Sepsis and Septic Shock (Sepsis-3) eliminated the term “severe sepsis” (Singer et al., 2016; RHODES et al., 2017). Sepsis was defined as “life-threatening organ dysfunction caused by a dysregulated host response to infection” (Singer et al., 2016; RHODES et al., 2017). This new definition focuses on the early detection of sepsis and the identification of patients requiring immediate intervention.

Currently, diagnostic methods for sepsis can take days to provide accurate results, delaying treatment. As a result, there has been a continuous search for faster, more sensitive, and specific biomarkers for sepsis detection. The expression of HLA-DR in monocytes (mHLA-DR) and CD64 in neutrophils (nCD64) —determined by flow cytometry—has been considered a reliable indicator of immunosuppression in critically ill patients. These parameters may be useful to diagnose sepsis (Monneret et al., 2006; Livaditi et al., 2006; Genel et al., 2010). A study conducted by Pradhan et al. (2016) suggested the possibility of establishing a “sepsis index” based on the ratio of the mean fluorescence intensity (MFI) of the two markers (Pradhan et al., 2016).

Our aim was to evaluate the efficacy of flow cytometry for assessing the expression of mHLA-DR, nCD64, and the sepsis index (SI) as laboratory tests in infection assessment. The cut-off points, sensitivity, and specificity of the variables for sepsis diagnosis were assessed. The objective was to validate the use of HLA-DR and CD64 biomarkers, along with the sepsis index, as prompt diagnostic tools for sepsis, contributing to the improvement of clinical outcomes for patients.