Significance of low level infliximab in the absence of anti-infliximab antibodies

Figure 3 Re-testing of double negative sera to increase analytical sensitivity. To examine whether some of the double negative sera represented low-titer antibodies to infliximab or low-level infliximab, we randomly selected 45 double negative sera and re-tested them at a 1:10 dilution to increase analytical sensitivity (compared with standard anti-lambda testing using 1:100 serum dilution). A: IFX and ATI values of double negative vs healthy controls’ sera, analyzed at 1:100 dilution anti-lambda ELISA. Cut off values for double negativity: IFX < 1 μg/mL, ATI < 2.1 μg/mL-eq. DN sera - black squares, healthy controls - circles; B: IFX and ATI values of sera DN on 1:100 dilution vs healthy controls analyzed at 1:10 dilution anti-lambda ELISA. Cut off values for double negativity: IFX < 0.04 μg/mL, ATI < 0.6 μg/mL-eq. Previously DN sera (on 1:100 dilution) - black squares, healthy controls - circles. DN: Double negative; IFX: Infliximab; ATI: Antibodies to infliximab.