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In their letter, Bendtzen et al1 question the methodology for measuring trough levels of anti-tumour necrosis factor (TNF) and antibodies. The authors state that the use of solid-phase assays (ELISAs) is inferior to the use of soluble-phase radioimmunoassays (RIAs) for measurements of antibodies to infliximab (ATI). We are grateful for the opportunity to raise a couple of points on this issue.

A number of recent publications have shown that in the treatment of patients with the anti-TNF antibody infliximab (IFX), detectable trough IFX serum levels predict clinical remission whereas undetectable trough IFX serum levels are associated with less favourable outcomes.2 3 In the study by Seow et al,3 the clinical outcome was the same irrespective of the presence of antidrug antibodies (ATI). The authors hypothesised that early formation of immune complexes in acute ulcerative colitis might result in rapid clearance of IFX. It is indeed possible that early rapid clearance is the consequence of early antibody formation. In this situation, the detection of both trough IFX serum levels and ATI serum levels depends greatly on the time point of sampling and on the ability of the used assay to detect ATI in the presence of IFX. The …