Unraveling the clinical significance and prognostic value of the neutrophil-to-lymphocyte ratio, platelet-to-lymphocyte ratio, systemic immune-inflammation index, systemic inflammation response index, and delta neutrophil index: An extensive literature review.

In the field of critical care medicine, substantial research efforts have focused on identifying high-risk patient groups. This research has led to the development of diverse diagnostic tools, ranging from basic biomarkers to complex indexes and predictive algorithms that integrate multiple methods. Given the ever-evolving landscape of medicine, driven by rapid advancements, changing treatment strategies, and emerging diseases, the development and validation of diagnostic tools remains an ongoing and dynamic process. Specific changes in complete blood count components, such as neutrophils, lymphocytes, monocytes, and platelets, are key immune system responses influenced by various factors and crucial in systemic inflammation, injury, and stress. It has been reported that indices such as neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), systemic immune-inflammation index (SII), systemic inflammation response index (SIRI), and delta neutrophil index calculated using various ratios of these elements, are important predictors of various outcomes in conditions where the inflammatory process is at the forefront. In this narrative review, we concluded that NLR, PLR, SII, and SIRI show promise in predicting outcomes for different health conditions related to inflammation. While these tests are accessible, reliable, and cost-effective, their standalone predictive performance for a specific condition is limited.