Red Blood Cell Distribution Width Level as Reliable Diagnostic and Prognostic Biomarker in Septic Patients: A Case-Control Study Background and Aim: Sepsis is one of the leading causes of death, with 40–50% mortality rates. The red blood cell distribution width (RDW) is an emerging, novel, and inexpensive marker for sepsis.This study aimed to evaluate RDW levels as a reliable diagnostic and prognostic biomarker in septic patients. Patients and Methods: A prospective case-control study was carried out in the Medical Intensive Care Unit (MICU), Zagazig University Hospitals on 46 patients, equally divided into two groups (n=23): the septic group and those who didn’t develop sepsis (control group). All participants underwent a C-reactive protein (CRP, mg/ml) and a complete blood count (CBC) analysis, including RDW, on the first, third, and seventh days after admission. Procalcitonin (ng/ml) was determined using an enzyme-linked immunosorbent assay. Results: Diabetes was the most common chronic disease, while lung infections were the most common source of sepsis. Septic patients had significantly higher RDW values compared to control patients on the first (18.9 vs. 13.8), third (20.4 vs. 14.3), and seventh day (25.7 vs. 16.0) after admission, p <0.05. RDW >14.8, CRP >38.7mg / l, or CRP >38.7mg/l, or procalcitonin >2.2 mg/l were correlated with development of sepsis. On the seventh day of admission to the ICU, the mortality rate was 43.5% and 13.1% in the septic and control groups, respectively. A positive correlation was detected between RDW and Sequential Organ Failure Assessment (SOFA) score on the seventh day of hospitalization (P<0.0001). Conclusions: RDW is significantly higher in septic and dead patients; hence, it may be considered an effective biomarker for early sepsis detection and reliable predictor of mortality in septic patients. .