The aim of this study was to examine the extent to which infliximab (IFX) serum levels impact disease activity in rheumatoid arthritis (RA) patients.
In this cross sectional study, serum samples were taken prior to drug infusion from 60 RA patients who had been undergoing IFX therapy > 12 months as a first line of biological treatment. Patient IFX levels were tested and then associated with clinical disease activity. Three DAS28 cut-off points, <2.6, <3.2 and <5.1 were used to determine whether detectable IFX levels were any predictor of clinical disease activity. Logistic regression analysis was run to check other possible factors associated with RA clinical outcomes such as MTX concomitant use, CRP and ESR.
Sixteen (27%) out of the 60 patients tested negative; 28 (46%) presented subtherapeutic and 16 (27%) therapeutic IFX levels. Median IFX levels were higher in patients either in remission or showing low disease activity than in those with moderate and high disease activity (p=0.014). Significant association was found between IFX levels and clinical disease activity (p=0.001). Detectable levels of IFX shows better sensitivity and specificity to identify patients with DAS28<3.2 than to identify patients with DAS28<2.6 or DAS28<5.1. Conversely, the best DAS28 cut-off to identify detectable/undetectable IFX was 3.19, with AUC under ROC curve 0.804 (Sd.E 0.070), 76% specificity and 83% sensitivity (p<0.001). MTX use, CRP and ESR did not interfere with this association. Seven out of the 8 patients with anti-IFX antibodies presented DAS28>3.2 (p=0.005).
DAS28 and IFX serum levels were shown to have an inverse correlation. Undetectable IFX serum levels were associated to RA patients presenting DAS28>3.2 meaning that DAS28 <3.2 may be useful to clinicians to evaluate patient response to drug therapy.