Sepsis is a challenging condition in which hematological prognostic and diagnostic markers in cats are limited. The aims of this study were to test if there are any differences in leukocyte ratios (NLR, BLR and BNLR) between healthy, SIRS and septic cats (sick cats), and if, within sick cats, NLR, BLR and BNLR may be prognostic markers. A retrospective medical database study included 76 healthy cats (blood-donors), 54 SIRS and 79 septic cats. SIRS group was defined if cats fulfilled SIRS criteria. Sepsis was confirmed with an infectious focus on cytology or a positive culture for bacterial infection. Leukocyte ratios were compared among the three study groups and between survivors and non-survivors in sick cats. NLR resulted significantly higher in the sick group compared to healthy cats ( < 0.0001), although NLR was not different between SIRS and sepsis. An NLR > 4.53 had a sensitivity of 76% and a specificity of 93.4% to detect SIRS/sepsis (OR 44.8 95%CI 17-107). Only BLR and BNLR were significantly different between SIRS and sepsis. NLR was associated with mortality in the sick group ( = 0.04). Although NLR resulted higher in sick cats than healthy, BLR and BNLR demonstrated as promising tools in differentiating SIRS from sepsis. NLR was associated with mortality in sick cats.