Current serological testing detects the IgA isotype of the Anti-TTG antibodies. Hence if a person is IgA deficient they will lack the prerequisite antibodies for testing despite the possibility of Celiac disease. The IgG class of anti-tTG may be ordered as an alternative in those who have a deficiency of IgA. Approximately 2-3% of the time in people with Celiac disease there is concurrent IgA deficiency and can lead to false-negative test results. 
1. Rittmeyer, C., Rhoads, J. M. 1996. IgA deficiency causes false-negative endomysial antibody results in celiac disease. J. Pediatr. Gastroenterol. Nutr. 23:504-506.