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What is significant autoantibody level?

Q At what level does an autoantibody titre become significant?

A As with so many things in life the significance of the titre of an antibody test depends upon the context. Antibody tests are best requested only when there is a strong clinical case for believing they might be helpful in making or excluding a diagnosis.

Consider the widely-tested anti-nuclear antibody (ANA) for example. The test is helpful in distinguishing those patients with Raynaud's phenomenon who have an underlying autoimmune rheumatic disease from those ­ the majority ­ who do not.

When asking your lab to look for it you must remember around 5-10 per cent of perfectly healthy people can have raised titres (up to about 1:160), especially if they have a close relative with systemic lupus erythematosus. But if a patient complaining of a photosensitive rash and/or arthritis is found to have an ANA of 1:80, that result is likely to be highly significant. Likewise, low-titre antibodies of anti dsDNA or anti cANCA (in the case of Wegener's granulomatosus) or anti-mitochondrial antibodies (in the case of primary biliary cirrhosis) in the appropriate clinical setting are likely to be highly significant. So there is no simple level at which an antibody titre becomes 'significant'.

Remember also around 30 per cent of myeloma proteins are autoantibodies so that a patient could have 20g/l of rheumatoid factor without any clinical evidence of rheumatoid arthritis.

Professor David Isenberg,

Centre for Rheumatology,

University College London

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