Tehran University of Medical Sciences Acta Medica Iranica 0044-6025 62 6 2024 12 31 338 345 Sura Mohammed Lateef Department of Microbiology, Al-Nahreen University, Collage of Medicine, Baghdad, Iraq. Ahmed Abdul-Hassan Abbas Department of Microbiology, Al-Nahreen University, Collage of Medicine, Baghdad, Iraq. Mohammed Hadi Alosami Department of Microbiology, Al-Nahreen University, Collage of Medicine, Baghdad, Iraq. 2024 10 03 2024 11 09 Rheumatoid arthritis is a chronic condition, characterized by the expression of antibody against self-antigens. Inflammatory cell of synovial tissues secreted numerous cytokines, include tumor necrosis factor alpha. Infliximab was designed to treat several autoimmune diseases. However, a considerable number of patients fail to respond appropriately. To investigate the relationship between trough infliximab serum levels and infliximab antibody with C-reactive protein (CRP), Rheumatoid factor (RF), and Anticyclic citrullinated peptide (ACCP) status. This study examined 83 rheumatoid arthritis patients on infliximab for 6 months duration. Sampling was collected in Rheumatology Unit at (Baghdad Teaching Hospital) from September 2021 to April 2022. Each patient had 3 mL of blood drawn. before the next dosage of medication. Anti-infliximab antibody, trough infliximab, and ACCP levels were measured using ELISA, while RF and CRP status were determined using an agglutination test. Ages of the patients ranged between 30-75 years old, (66 females and 17 males), and 44.6% of patients were smokers. In this study, 41 out of 83 patients were responder for infliximab therapy. The seropositivity of anti-drug antibodies was 50% in non-responder and 48.80% in responder patients. In contrast, the infliximab trough level classification as low and high in responder and non-responder patients was high in 56.1 %, and 40.5% of patients respectively. The difference between the two groups was statistically non-significant P=0.114. In addition, 74 patients tested positive for anti-cyclic citrullinated peptide. (89.2%), abnormal CRP levels were found in 54 (65.1%) patients, and seropositivity of Rheumatoid factor was found in 52 (62.7 %) of the patients. There was a negative relationship between smoking and responsiveness to infliximab. (r= -0.295 P 0.007) while there was a positive correlation between anti-infliximab antibodies with CRP and RF (P=0.026). Likewise, ACCP and serum trough infliximab levels were correlated significantly (P=0.014). Antidrug antibody seropositivity correlates positively with CRP and RF status and between ACCP and serum trough infliximab in RA patients while there is a negative correlation between smoking and early response to infliximab. https://acta.tums.ac.ir/index.php/acta/article/view/11278