Objective To investigate the changes in serum complement activation product(sC5b-9),serotonin(5-HT),and peripheral blood C1q mRNA levels in patients with bipolar disorder during manic episodes,and their correlation with cognitive impairment.Methods From January 2022 to December 2023,75 patients with bipolar disorder during manic episodes and 75 healthy individuals who were treated at Baoji Central Hospital were selected and divided into BD group and healthy group.Serum sC5b-9,5-HT,peripheral blood C1q mRNA levels,and Montreal Cognitive Assessment(MoCA)scores were detected and compared.Patients in the BD group were further grouped based on MoCA scores:cognitive impairment group(22～25 points)(n=40)and non-impairment group(≥26 points)(n=35).Differences in serum sC5b-9,5-HT,and peripheral blood C1q mRNA levels were compared,and multiple Logistic regression analysis was performed to identify influencing factors of cognitive impairment in bipolar disorder patients.Results Compared to the healthy group,the BD group showed significantly elevated serum sC5b-9,5-HT,and peripheral blood C1q mRNA expression levels(P<0.05).The BD group's 8 MoCA domain scores and total MoCA score were significantly lower than the healthy group(P<0.05).Compared to the non-impairment group,the cognitive impairment group demonstrated significantly higher serum sC5b-9,5-HT,and peripheral blood C1q mRNA expression levels(P<0.05).Serum sC5b-9,5-HT,and peripheral blood C1q mRNA expression levels were risk factors for cognitive impairment in patients with bipolar disorder during manic episodes(P<0.05).Conclusion Patients with bipolar disorder during manic episodes show abnormally elevated serum sC5b-9,5-HT,and peripheral blood C1q mRNA expression levels.Cognitive impairment in these patients is closely related to the expression levels of serum sC5b-9,5-HT,and peripheral blood C1q mRNA.