The Surviving Sepsis Campaign (SSC) of 2021 characterizes sepsis as a life-threatening organ dysfunction caused by a dysregulated host response to infection, with a mortality rate of over 30%, which increases to 40% or more in the case of septic shock. Although concept of sepsis has improved, significant gaps remain in assessing its clinical severity and predicting patient outcomes. Recognized markers of the severity of septic shock are procalcitonin (PCT), presepsin, and, to a certain extent, lactate. Elevated lactate levels result from a shift to anaerobic glycolysis and indicate inadequate oxygen delivery to tissues. Despite the importance of procalcitonin in the course of sepsis, it has proven to be a suboptimal diagnostic biomarker for the development of sepsis, with sensitivity and specificity below 80%. As a result, the use of procalcitonin in addition to clinical assessment to determine the indications for initiating antibiotic therapy is not recommended. Meanwhile, presepsin, although a potential biomarker for the early diagnosis of sepsis, also has only moderate accuracy according to current data and cannot be used as the only test for the diagnosis of sepsis. Renin is a crucial enzyme in the renin-angiotensin-aldosterone system (RAAS), which plays an important role in the regulation of blood pressure, tissue perfusion, and the balance of water and electrolytes. Thus, renin may be the sensitive biomarker with good prognostic qualities in the context of sepsis and septic shock. Preliminary studies have indicated that elevated renin levels may act as an indicator of patient severity, and could also be used as a marker for the development of septic shock and mortality prognosis. This study aims to address these gaps by investigating the role of renin as a biomarker for the severity of sepsis and septic shock, focusing on its potential for more accurate prediction of clinical outcomes.