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

AIM:To evaluate the prevalence of double negative(DN)sera and the mechanisms responsible for DN status.METHODS:Sera of inflammatory bowel disease patients treated with infliximab(IFX)were tested for drug/antibodies to infliximab(ATI)trough levels and the proportion of DN results was compared between a commercially available double antigen ELISA(with labeled IFX as the detection antibody)and an antilambda ELISA(with anti-human lambda chain detection antibody).Repeat testing with lower than customary serum dilution(1:10)was performed.Patients with DN status were matched with IFX+/ATI-controls and were followed-up for subsequent development of nontransient ATI to investigate if DN status precedes ATI.RESULTS:Of 67 sera obtained at time of loss of response,only 6/67(9%)were DN by anti-lambda ELISA compared to 27/67(40%)with double antigen ELISA(P<0.001,Fisher’s Exact test).Of the latter27 sera,22%were also DN by anti-lambda ELISA,whereas 44%were actually IFX positive(IFX+ATI-),30%were ATI positive(IFX-ATI+)and 4%were double positive(IFX+ATI+).Re-testing using a 1:10 dilution converted most DN results into IFX+and/or ATI+status.Patients with DN status had shorter survival free of non-transient ATI compared with matched controls(log rank test,P<0.001).In 9/30(30%)of these patients,non transient ATI occurred before and after the event at which the DN serum was obtained,supporting the view that a DN result may represent aparticular time-point along the two curves of ATI titer rise and infliximab drug level decline.CONCLUSION:DN status may result from false negative detection of IFX or ATI by double antigen ELISA,suggesting a transitional state of low-level immunogenicity,rather than non-immunological clearance.