Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1254 page)

   Certain drugs (e.g., androgens, anabolic steroids; glucocorticoids [TBPA is increased])

   Testosterone-producing tumors

   Major illness, surgical stress, protein malnutrition, malabsorption resulting from various causes

   Limitations

   Decreased binding of T
₃
and T
₄
due to drugs (salicylates, phenytoin, Orinase, Diabinese, penicillin, heparin, barbital)

TISSUE TRANSGLUTAMINASE IgA ANTIBODY (tTG-IgA)

   Definition

   Celiac disease (CD) is an immune-mediated enteropathy caused by a permanent sensitivity to gluten in genetically susceptible individuals. Testing should begin with serologic evaluation, and the most sensitive and specific tests are tissue transglutaminase IgA antibody (tTG-IgA) and endomysial IgA antibody (EMA-IgA), which have equivalent diagnostic accuracy. Anti-tTG antibodies are highly sensitive and specific for the diagnosis of CD. The enzyme tTG is the major target antigen recognized by anti-endomysial antibodies. Based on the current evidence and practical considerations, including accuracy, reliability, and cost, measurement of tTG-IgA is recommended for initial testing for CD. Although as accurate as tTG, measurement of IEMA-IgA is observer dependent and, therefore, more subject to interpretation error and added cost. Because of the inferior accuracy of the antigliadin antibody (AGA) tests, the use of AGA IgA and AGA IgG tests is no longer recommended for detecting CD.

   
Normal range:
<20 U (negative).

   Use

   Diagnosis of certain gluten-sensitive enteropathies, such as CD and dermatitis herpetiformis

   Monitoring adherence to gluten-free diet in patients with dermatitis herpetiformis and CD

   Evaluating children with failure to thrive

   Interpretation