Tests for IgA Antibodies to Tissue Transglutaminase Vary Too Much For Easy Commutability

Celiac.com 08/26/2015 - People with IgA antibodies to tissue transglutaminase (anti-tTg) likely have a higher risk for celiac disease. Some clinicians and researchers have suggested that common multiples of the upper limit of normal (ULN) be useful tool in improving diagnostic pathways, as well as continuity between tests.

However, a new study suggests that both sensitivity and specificity of tests for IgA antibodies to tissue transglutaminase vary widely by individual kit, and that their test values are not easily commutable using common multiples of the ULN to correct for inter-assay variations. Commutability just means the ability to make sure that two different tests really are equal. If results of different tests are commutable, it means that they are equal. In this case, the term applies to test results for various representative samples from healthy and diseased individuals.

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For the study, the research team recently looked at the use of immunoassays for the detection of IgA antibodies to tissue transglutaminase, and also sought to better understand of the significance of multiples of the upper limit of normal and inter-assay correlations. The research team included B.B. Suh-Lailam, K.W. Davis, and A.E. Tebo. Using indirect immunofluorescence assay (IFA) as reference, the team assessed characteristics of four anti-tTG IgA assays relative to endomysial IgA (EMA).

They also assessed commutability between anti-tTG immunoassays and/or EMA based on manufacturer's recommended cut-off values and three common multiples of ULN (3×, 5× and 10×). To do this, they analyzed samples from 200 patients and 100 healthy individuals.

They found that, at manufacturer's cut-off, the sensitivities for the tTG assays ranged from 72.5% to 98.6% and specificities from 60.3% to 99.2%. The percent positive agreements between any anti-tTG and EMA or any two anti-tTG immunoassays varied from 56.7% to 98.0% and 46.7% to 100.0%, respectively.

At 3×, 5× or 10× ULNs, the inter-rater reliability as measured by Cohen κ between any two anti-tTG assays were quite variable and ranged from 0.28 to 0.96, 0.26 to 0.89 or 0.13 to 0.78, respectively.

Furthermore, the percent positive agreements between any two anti-tTg IgA immunoassays ranged from 83.1% to 98.2%, 92.0% to 100%, or 100%, at 3×, 5× or 10×, respectively.

Hence, the team's basic takeaway that result parameters for tTG IgA immunoassays or tTG IgA and EMA vary by kit, and thus common multiples of the ULN are not enough to correct for variation between tests.

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