Neutrophil-to-lymphocyte ratio-a new diagnostic and prognostic marker of acute kidney injury. Barriers to broad clinical application.

Hospital-acquired acute kidney injury is a heterogeneous clinical syndrome that has multiple aetiologies, widely differing pathogeneses, variable clinical manifestations, and diverse outcomes. There is a persistent unmet need for novel biomarkers that offer timely diagnosis and accurate prediction of the short- and long-term sequelae of acute kidney injury (AKI). AKI is associated with systemic and intrarenal inflammation. The neutrophil-to-lymphocyte ratio (NLR), a readily available marker of inflammation and physiologic stress, has gained increasing attention as universal marker in AKI patients. Numerous retrospective cross-sectional studies assessed the clinical usefulness of this test in high-risk patients with a known time point of the renal injury (surgery, radiological procedures). Strong associations have been demonstrated between high NLR and early onset, progression or recovery of AKI, and the in-hospital and post-discharge mortality of these patients. However, the results were contradictory. The huge heterogeneity of reporting concerning the timing and numbers of blood samples, calculation of the optimal cut-off and the demographic and clinical features of the patient cohorts were confounders. Uncertainty in the optimal cut-off values defining high NLR, the lack of prospective validation of this test and limited understanding of the strengths of associations between NLR and clinical outcomes were further barriers for the clinical adoption of NLR as a valid diagnostic and prognostic test in AKI patients.