Blood tests for diagnosis include several alternatives suited for specific circumstances. IgA deficiency, a rare condition affecting a few people with celiac disease, causes undetectable IgA anti-TTG. This can call for an IgA deficiency test and alternate tests for celiac disease based on immunoglobulin G (IgG). However, their accuracy is less well known.
Genetic tests for celiac disease are now being commonly used. The human leukocyte antigen (HLA) gene codes for an agent in the immune system. Two forms, HLA-DQ2 and HLA-DQ8, occur in about 40 percent of the population. Only people who carry these genes can develop celiac disease: Still, the vast majority never will.
Some celiac disease experts have found the problem with HLA tests is that patients and many doctors interpret a positive result as strong evidence for a diagnosis, which it is not. It indicates only that the patient has a higher risk for celiac disease. The ESPGHAN guidelines recommend including it with other blood tests in reaching diagnosis, even though genetic testing is inconclusive and relatively expensive to perform.
A negative HLA result is helpful in ruling out celiac disease in the majority of cases. The ACG recommends this as a first step for patients who have already adopted a gluten-free diet without diagnosis. A negative result indicates the diet is unnecessary. However the genetics of celiac disease are not entirely understood, and about 1 percent of patients have none of the gene types so far identified. These rare individuals would get a negative HLA result.