Abstract :Objectives: The main assay recommended in chronic urticaria is complete blood count (CBC). It is known that changes in the numerical values and rates of blood cells in the CBC are associated with inflammation and thrombotic risk. In addition, the improvement of some patients with short-term Omalizumab treatment, while others need long-term treatment raises the question of whether this difference can be predicted with parameters on the CBC. This study aimes to evaluate the effect of the drug on CBC parameters in patients with chronic spontaneous urticaria (CSU) receiving Omalizumab treatment. Methods: The study included 66 patients who received Omalizumab treatment with CSU and 34 healthy individuals as a control group. The values of 17 parameters from the biomarkers in the CBC were recorded in the patient group (before the treatment/after 12 weeks of treatment) and in the control group. In addition, patients with CSU according to the Omalizumab treatment requirement was divided into two groups as those who recovered in a short time (≤6 months) (group-1) and those who need longer treatment (> 6 months) (group-2). All data were compared statistically. Results: It was observed that many inflammatory and thrombotic activation markers in the CBC increased in the patient group before treatment compared to the control group, and even most of these values approached the values of healthy individuals after treatment. When the pre-treatment and post-treatment data of the patients are compared within themselves; neutrophil and platelet numbers and neutrophil-monocyte ratio decreased after treatment and there was a statistically significant difference. When the pre-treatment values of the two groups determined according to the treatment requirement period were compared, it was determined that the eosinophil-basophil ratio (EBR) value was significantly higher in the group-2 patients. It was observed that the number of platelets in group-1 decreased and eosinophil-lymphocyte ratio and eosinophil-neutrophil ratio increased after treatment compared to pre-treatment; platelet, neutrophil counts and EBR values of patients in group-2 decreased, mean platelet volume (MPV) and platelet distribution width (PDW) values increased. Conclusion: It has been determined that Omalizumab treatment has positive effects on patients with CSU that it reduces many inflammatory and thrombotic activation markers in CBC. It was thought that an increase in EBR may be a biomarker that predicts that patients with CSU will need Omalizumab treatment for a longer period. In addition, it was concluded that MPV and PDW values increased during Omalizumab treatment of patients with long-term treatment needs, and that thrombotic activation markers of these patients should be followed more closely during the treatment. Keywords : Chronic spontaneous urticaria, Omalizumab, , complete blood count, biomarker, thrombotic activation.