Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1255 page)

Increased In

   CD (20–30 U: weak positive; >30 U: moderate to strong positive)

   Autoimmune skin disease and dermatitis herpetiformis

   Limitations

   All testing should be performed while patients are on a gluten-containing diet.

   IgA deficiency is more common in CD (2–5%) than in the general population (<0.5%). The EMA-IgA and tTG-IgA serology tests will be falsely negative in untreated CD in patients with IgA deficiency. As a result, total serum IgA can be measured in addition to EMA-Ig A or tTG-IgA, especially when there is heightened clinical suspicion for CD and IgA markers are negative. If total IgA levels are abnormally low, an IgG-based assay should be used to test for CD.

   The IgG antigliadin assay has been traditionally used in this circumstance but is not ideal, since it yields frequent false-positive results. Therefore, serum IgG-tTG or IgG deamidated gliadin peptide (DPG) tests are preferable. Negative results on testing for HLA DQ2 or DQ8 can also help exclude the diagnosis in this setting.

   If serology is negative and/or there is substantial clinical doubt remaining, then further investigation should be performed with endoscopy and bowel biopsy. This is especially important in patients with frank malabsorptive symptoms, since many syndromes can mimic CD. For the patient with frank malabsorptive symptoms, bowel biopsy should be performed regardless of serologic test results.

   False-positive tests are rare but have been reported in patients with other autoimmune syndromes. Because the tTG antigen is derived from liver cells, false-positive results may be seen in patients with autoimmune liver disease.

Suggested Reading

Hill ID, Dirks MH, Liptak GS, et al. Guideline for the diagnosis and treatment of celiac disease in children: recommendations of the North American Society for Pediatric Gastroenterology, Hepatology and Nutrition.
J Pediatr Gastroenterol Nutr.
2005;40(1):1–19.

TRANSFERRIN (TRF)

   Definition

   Transferrin transports circulating Fe
³⁺
molecules. Normally only about one third of iron-binding sites are occupied (the remainder is called unsaturated iron-binding capacity). The half-life of transferrin is approximately 8–10 days. Plasma levels are regulated primarily by availability of iron, iron deficiency anemia. Plasma levels rise on successful treatment with iron and return to normal.

   
Normal range:
202–336 mg/dL.

   Use