Abstract Background Approximately half (10-40%) of the Inflammatory Bowel Disease (IBD) patients using Infliximab (IFX) develop secondary loss of response due to drug levels falling below the therapeutic range. This study aims to identify drug levels, influencing factors and parameters that may be useful in predicting target drug levels. Methods The pre-infusion drug levels and associated laboratory parameters were evaluated in IBD patients on maintenance dose of IFX (5-15 mg/kg every 4-8 weeks) between Nov2021 - Oct2024. Drug levels were measured using a bedside kit capable of quantitatively detecting values between 0.4-20 µg/ml. Results In total, 931 single pre-infusion measurements from 415 different patients, and dual measurements at weeks 4 and 8 from 116 patients were separately evaluated. 82% of the patients in the single measurement group, and 84% in the double measurement group had Crohn’s disease(CD), rest having Ulcerative Colitis. The most used doses in the single measurement group were 5 mg/8 weeks and 10 mg/4 weeks (Table 1). In the single measurement group, median albumin was 4.4 (IQR: 0.5), and median CRP was 3.1 (IQR: 6.4). Drug levels were below 5 µg/ml in 33% of measurements. CRP showed a significant difference according to drug level groups (Table 2), while no such difference was observed for albumin. In the measurable IFX range (0.4-20 µg/ml), correlation analysis revealed a weak negative correlation between CRP and drug levels (r: -0.09, p: 0.03). Regression analysis showed that lower CRP, higher drug dose, and CD diagnosis were independently associated with achieving >5 µg/ml drug levels. In the dual measurement group: The median CRP levels of patients at weeks 4 and 8 were 2.1 (5.1) and 2.6 (6.2), respectively (p>0.05). Among patients with IFX levels >20 µg/ml at week 4, 79% maintained levels >5 µg/ml at week 8, while this rate dropped to 71% for those with levels >10 µg/ml. Only 67% of patients with levels >5 µg/ml at week 4 maintained a level of >5 µg/ml at week 8 (all p-values <0.001). Conclusion As drug levels increase, significant reductions in CRP levels are observed. Therefore, low CRP levels may predict the likelihood of achieving the therapeutic range. Dose adjustment should be considered in cases with high CRP levels. Achieving a high drug level (>20 µg/ml) at 4th week is an important indicator of achieving at least an acceptable drug level (>5 µg/ml) at the end of treatment. (8th week).
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