Highs and lows of coeliac screening

Coeliac screening is one of the most frequently requested investigations in immunology laboratories. This study evaluates seven commercial anti-tissue transglutaminase (tTG) enzyme-linked immunosorbent assay (ELISA) kits across a varied population of 80 routine samples received for coeliac screening. This investigation assesses whether or not absorbance levels can be used to detect low serum IgA and whether or not raised IgA concentrations can interfere with IgA-tTG results. Sensitivity and specificity varied hugely and four out of the seven methods required cut-off value modification to attain 100% sensitivity with endomysial testing. Four of the seven kits identified low IgA samples with greater than 90% accuracy, but sensitivity dropped to 75% in others. All the kits were affected by raised serum IgA concentrations, leading to false-positive results. There was notable variation between the seven kits in respect of high IgA concentrations and positive IgA-tTG results, with concordance analysis indicating a weak linear relationship between IgA concentration and tTG value. This study concludes that there is significant variability between the commercial tTG assays in the diagnostic market. Laboratories should be aware of their kit's limitations and may need to adjust cut-off values to maximise sensitivity. It is possible to identify IgA deficiency from the tTG values, but the ability to do this varies between manufacturers. Raised IgA levels continue to affect the specificity of IgA-tTG assays and interference by polyclonal and monoclonal IgA should be considered in samples with positive tTG and negative endomysial results.