The measurement of serum concentrations of infliximab (IFX)has now become a routine part of optimal use of that drug. Trough values are used in two situations: (a) reactively where there is loss of response to infliximab - therapeutic concentrations are indicate likely non-response to the drug, whereas low levels are associated with the chance of regaining response by increasing dosage; or (b) proactively, where dose optimisation in the maintenance phase is performed to ensure ongoing efficacy and/or cost-effective use (where high levels lead to reduction in dosage without loss of efficacy).||With the introduction of biosimilar infliximab into clinical practice, it is important to demonstrate that the biosimilar behaves similarly in the assay used as does originator infliximab to which the assays were developed. While unlikely to be different due to identical protein core, such confirmation is needed before such assays can be used in clinical practice with confidence.||AIMS||To compare the concentrations of biosimilar IFX (MSD-IFX) with that of originator IFX (orig-IFX) when added to serum form healthy subjects and those with IBD when measured by commonly-used commercial assays.|To compare the effect of freeze-thawing and storage at 4 oC on concentrations of MSD-IFX.