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We feel we should comment on a recent article by Iwanaet al 1 on the clinical value of measuring circulating IgA-αl-antitrypsin (IgA-AT) complex concentrations in patients with rheumatoid arthritis (RA) using a prototype ELISA kit. We are concerned about the specificity of the monoclonal antibody used as the capture reagent on their ELISA plates. The authors say that the antibody recognises specific epitopes on the IgA-AT complex. However no direct proof of this is provided here or in previous reports where this particular antibody has been used.2-6 Recently, in response to another study using this assay we provided data to show that the antibody recognises the complement regulatory protein, factor H.7 We have shown that replacing the ‘complex specific’ antibody with other monoclonal antibodies to factor H (OX23 and OX24) in the ELISA essentially makes no difference to measurement of ‘complex’ values. We have also shown that the ‘lgA-AT’ antibody recognises a different epitope on factor H to that recognised by OX23 and OX24, and feel that …