What does my coeliac disease blood test actually mean?

Gluten-free diets have become very popular and you may have wondered, “could I have coeliac disease?”. In Europe, coeliac disease affects approximately 1 in every 100 people¹ and is common across all ages. However, symptoms of coeliac disease can be vague and many people with coeliac disease are thought to be undiagnosed^{2, 3}. Testing for coeliac disease can be complicated, but we’re here to simplify it. We need to understand what coeliac disease is and what causes it so we can make sense of the tests. That’s where we’ll begin.

Coeliac disease is an “autoimmune condition” which means that it’s caused by your immune system reacting against your own body. In coeliac disease, a substance in your diet called gluten triggers your immune system to attack the lining of your gut, causing inflammation. Gluten itself is harmless, it’s the body's reaction to it that causes a problem. Gluten is a protein found in wheat, barley and rye which makes dough stretchy. It’s found in a lot of common foods like bread, pasta, flour and even beer and soy sauce. If you have coeliac disease and eat gluten, your body sees it as a threat and tries to destroy it, damaging your gut in the process (figure 1).

A specific part of gluten called “gliadin” is the culprit here. In coeliac disease, the body confuses gliadin for something dangerous, like bacteria, which triggers an immune response. This immune response involves the body's defence system using white blood cells and antibodies designed to attack the gliadin. Unfortunately, in coeliac disease the antibodies can also target the gut lining, causing inflammation.

The important point to take away here is that coeliac disease specifically involves inflammation in the gut. There is another condition where gluten can cause gut symptoms, called non-coeliac gluten sensitivity. As the name suggests, this is not coeliac disease. It’s a gluten “intolerance” and there is no inflammation. It’s not an autoimmune disease.

Antibodies are proteins in your blood produced by your white blood cells. Antibodies can stick to other cells - like bacteria - and flag them for attack by your immune system. Antibodies have a specific shape which means they only stick to things of the corresponding shape. A bit like how a key will only fit a specific lock. There are a couple of different types of antibodies. Two common ones are called “IgG” and “IgA”.

As we’ve said, antibodies produced by an immune response have a specific shape depending on what they’re targeting. People with coeliac disease make specific-shaped antibodies that attack certain parts of the gut lining. We can use a blood test to pick them up. In general, only people who have coeliac disease will have high levels of these antibodies so if we find them then it’s likely that you have the condition. On the other hand, if we don’t find them, then it’s unlikely you have coeliac disease.

There are a couple of antibodies we can use to test for coeliac disease. One of the most useful is an IgA type antibody that sticks to a protein in your gut called tissue transglutaminase. We call this antibody “TAA” for short.

Another antibody used in testing sticks to a protein called endomysium which makes up part of the gut lining. It’s another IgA type antibody and we call it “EMA” for short.

Lastly, there is also an IgG type antibody that’s helpful. These stick to gliadin itself and are called deamidated gliadin peptide (or DGP) antibodies.

All these antibodies (and others) have pros and cons when it comes to testing for coeliac disease but the TAA antibody probably works best and it’s the “first-line” test recommended by the National Institute for Health and Care Excellence (NICE) in the UK⁴.

Some people have very low or undetectable levels of IgA in their blood. This is called IgA deficiency and it’s not harmful in itself. The problem is that IgA deficiency is more common in people with coeliac disease. An Italian study with 2098 participants found that 13 in 500 people with coeliac disease also had IgA deficiency, compared with fewer than 1 in 500 people without coeliac disease⁵. This means it's more than 10 times as likely that you’ll be IgA deficient if you have coeliac disease.

As IgA deficiency means you have little or no IgA antibody, any coeliac blood tests looking for IgA type antibodies will be negative, even if you do have coeliac disease. To overcome this, we have to use the tests for IgG type antibodies in people with IgA deficiency.

You must be eating gluten in your diet before doing a coeliac blood test. Your antibodies against gluten are only detectable when you’re eating it in your diet. Even if you had coeliac disease and took a blood test, you’d get a negative result if you were on a gluten-free diet. In fact, doctors can actually use the test to check if people with coeliac disease are maintaining a gluten-free diet.

If you’re currently cutting out gluten, you’ll need to switch back for around 6 weeks before taking any blood tests and eat more than one gluten-containing meal each day. You should keep this up until your results are confirmed by a gut specialist doctor⁶.

If eating gluten isn’t possible for you, a coeliac blood test won’t work. There is a genetic test which can give some information about the likelihood of your symptoms being coeliac disease, or you could see a specialist to talk about other ways to diagnose coeliac disease⁶.

Our coeliac disease testing is based on guidance from NICE⁴ and follows the pathway outlined in figure 2 below. First, we test for TAA. If this is positive then it’s likely you have coeliac disease. In some cases we might then test for EMA and if this is positive too then you can be almost certain you’ve got the condition.

If your TAA level is normal - not too low and not too high - then no further testing is done. You’re unlikely to have coeliac disease.

However, a “very low” TAA level suggests that you might be IgA deficient so we then check your total level of IgA antibody. If this comes back as normal (no IgA deficiency) then we don’t do any other tests. If the total IgA level is very low then it would suggest that you have IgA deficiency and we do the IgG coeliac disease test (DGP). A positive DGP antibody test would make coeliac disease likely.

We’ll report your TAA levels as Negative, Positive or Very Low.

You have low levels of TAA antibodies. This is normal for people without coeliac disease. You can be confident that you don’t have the disease. No further tests are needed. Please note that if you’ve not been eating gluten in your diet for 6 weeks prior to testing, you could get a false negative result.

You have TAA antibodies and are likely to have coeliac disease. The next step would be getting a referral to a gut specialist doctor to potentially have a camera test to check the small intestine and discuss a gluten-free diet.

We couldn’t pick up any TAA antibodies at all. We will then test your total IgA level to make sure this isn’t a sign of IgA deficiency. If the total IgA level comes back as normal, then you don’t have IgA deficiency or coeliac disease either. If you do have IgA deficiency, then we’ll check for DGP IgG antibodies. If the DGP test is negative then you don’t have coeliac disease. If it’s positive then you’re likely to have coeliac disease, so you should see a gut specialist doctor. Please make sure that you were eating a gluten-containing diet before these tests were carried out, otherwise negative results may be wrong.

You can order our Coeliac Disease Gluten Sensitivity Test and get results back within a few days. This is a simple finger-prick blood test that you can do at home and post to our lab. You’ll get a doctor's report to make sure you understand your results.