Understanding the Role of IgA and tTG in Coeliac Disease Diagnosis
Introduction
In this week’s edition of The Celiac Scientist, I want to address a topic that often causes confusion in the Coeliac community: the difference between Immunoglobulin A (IgA) and tissue transglutaminase (tTG) in the diagnosis of Coeliac disease. I've seen many questions and misconceptions about these tests on social media, so let's break it down in a simple and informative way.
What is Coeliac Disease?
Coeliac disease is an autoimmune disorder where the ingestion of gluten leads to damage in the small intestine. Diagnosing Coeliac disease accurately is crucial to managing and treating it effectively.
IgA and tTG: What Are They?
Immunoglobulin A (IgA) is a type of antibody that plays a critical role in mucosal immunity, which includes the gut lining.
Tissue transglutaminase (tTG) is an enzyme found in various tissues, including the small intestine. The body produces antibodies against tTG (anti-tTG) when someone with Coeliac disease consumes gluten.
The Role of IgA and tTG in Diagnosis
tTG-IgA Test: The primary blood test for Coeliac disease measures IgA antibodies against tTG. This test is highly sensitive and specific. High levels of tTG-IgA antibodies usually indicate an immune reaction to gluten.
Total IgA Test: Since some people have IgA deficiency (a condition where the body doesn’t produce enough IgA antibodies), it’s crucial to measure the total IgA level. If someone has IgA deficiency, the tTG-IgA test might be falsely negative, even if they have Coeliac disease.
Common Questions Answered
1. Can you be Coeliac with a normal IgA but a high tTG result?
Yes, you can. If you have normal total IgA but high tTG-IgA, it strongly suggests Coeliac disease. The presence of high tTG-IgA indicates an autoimmune reaction to gluten.
2. Can you be Coeliac with both low IgA and tTG?
Yes, it’s possible, especially if you have IgA deficiency. In such cases, tTG-IgG (another type of antibody test) is often used instead. If both IgA and tTG-IgA levels are low, your doctor might look at tTG-IgG or other diagnostic tests like the endomysial antibody (EMA) test, which is less affected by IgA deficiency.
3. Can the blood test alone confirm you have Coeliac disease?
Blood tests are a critical first step but are insufficient for a definitive diagnosis. While high tTG-IgA levels are strongly indicative of Coeliac disease, a biopsy of the small intestine is usually required to confirm the diagnosis. The biopsy checks for damage to the villi in the small intestine, which is a hallmark of Coeliac disease.
Diagnostic Process
Blood Tests: Initial screening with tTG-IgA and total IgA tests.
Biopsy: If blood tests suggest Coeliac disease, an endoscopic biopsy is performed to look for villous atrophy.
Follow-up Tests: In cases of IgA deficiency, additional tests like tTG-IgG or EMA-IgA may be used.
References
Lebwohl, B., Sanders, D. S., & Green, P. H. (2018). Coeliac disease. The Lancet, 391(10115), 70-81. Link to study
Rubio-Tapia, A., Hill, I. D., Kelly, C. P., Calderwood, A. H., & Murray, J. A. (2013). ACG clinical guidelines: diagnosis and management of celiac disease. The American Journal of Gastroenterology, 108(5), 656-676. Link to guidelines
Conclusion
Understanding the difference between IgA and tTG in Coeliac disease diagnosis can help clarify the process and importance of each test. While blood tests are crucial for screening, a biopsy remains the gold standard for a definitive diagnosis. If you have any concerns or symptoms suggestive of Coeliac disease, it’s essential to consult with a healthcare professional for proper testing and diagnosis.
If you have more questions or topics you'd like me to cover, please let me know in the comments or message me. Your feedback is invaluable in making this newsletter helpful and relevant to you.
Stay healthy and gluten-free!
Warm regards,
The Celiac Scientist