Tissue Transglutaminase Antibody (TTG)
|Alternative name:||Coeliac Screen|
|Description:||IgA anti-tissue transglutaminase antibodies are now used as the preferred test for screening of coeliac disease and dermatitis herpetiformis, being highly (>90%) sensitive and specific. All positive samples are then tested for IgA anti-endomysial antibodies which are even more specific but though the test is technically more demanding and time consuming to perform. There is a good correlation between anti-TTg antibody and disease activity. Relapse or poor compliance with a gluten free diet is often associated with return of antibody positivity.The widespread use of these tests has led to the realisation that coeliac disease is common in all age groups even the elderly and presentation can be varied. An immune response to tissue transglutaminase or its products is the cause of coeliac disease. Most untreated coeliacs will have both IgA anti-tTg and endomyial antibodies. IgA anti-tTg tends to appear before anti-endomysial (sometimes before overt symptoms). On a gluten free diet IgA anti-tTg usually disappears after IgA anti-endomysial. There area few situations which lead to IgA anti-tTg positivity in the absence of coeliac disease, most commonly this is in chronic liver disease where total IgA levels are raised.
Note that coeliac disease is is often associated with IgA deficiency, a common immundeficiency found in around 1in 500 of our population. IgA levels are estimated in all patients with suspected coeliac disease. IgA deficient individuals with suspected coeliac disease are tested for IgG anti-tTg and IgG anti-endomysial antibodies. The test for IgA anti-TTg will usually detect IgA deficiency and indicate the need for measurement of IgG antibodies. Positive tTG and Endomysial are supportive of a diagnosis of Coeliac disease. A jejunal biopsy may be performed.
|Indication:||Highly sensitive and specific test for coeliac disease or dermatitis hepetiformis.|
|Interpretation:||Positive IgA anti-tTg is highly suggestive of coeliac disease. When IgA anti-endomysial antibody is also present, a diagnosis of coeliac disease is almost certain. A limited number of patients are positive for IgA anti-tTg but not IgA anti-endomysial Occasionally a positive IgA anti-tTg test can precede clinical symptoms. Chronic liver disease particularly where total IgA is raised can give positive IgA tTg (but seldom IgA anti-endomysial) in the absence of coeliac disease. Since this is an IgA assay, IgA deficient patients should have IgG anti-tTg and/or IgG anti-endomysial assay performed. Coeliac disease is more common in IgA deficiency.|
|Sample:||Serum Separator Tube (SST)|
|Assay details:||Fluorescence enzyme linked immunoassay (Phadia Immunocap 250) Human recombinant tissue transglutaminase : IgA (or IgG) antibodies.|
|Restrictions:||IgG anti-tTg only carried out on IgA deficient samples.|
|Assay range notes:||Result reported as POSITIVE/NEGATIVE.|
|Turnaround time:||5 – 7 days|
|Analysing laboratory:||Immunology The James Cook University Hospital|