In short
- Allergy is a reaction of the immune system through IgE antibodies. Intolerance is a problem with digestion or metabolism. The immune system is not involved.
- Specific IgE tells you whether you are sensitised. It does not tell you whether you will get symptoms after eating the food. A doctor makes the diagnosis from your history and the test together.
- IgG and IgG4 food tests are not diagnostic. European allergy experts advise against them. A high IgG shows that you eat the food, not that it harms you.
- Lactose intolerance is not measured with blood IgG. It is measured with a hydrogen breath test or a genetic test.
- Coeliac disease is measured with tTG-IgA together with total IgA — and only while you are still eating gluten.
Few areas are so full of tests that explain nothing. This article separates what blood can show and what it cannot. It also tells you which test is right for which question.
Allergy and intolerance are not the same
| Allergy | Intolerance | |
|---|---|---|
| Mechanism | Immune system, IgE antibodies. | Digestion or metabolism; the immune system is not involved. |
| How much is enough | Even a trace of the food can trigger a reaction. | Depends on the amount; a little often causes no trouble. |
| How fast | Minutes to two hours. | Hours, often the same day. |
| Signs | Hives, swelling, trouble breathing, vomiting; rarely anaphylaxis. | Bloating, belly pain, diarrhoea, gas. |
| Danger | Can be life-threatening. | Unpleasant, not life-threatening. |
| Blood test | Specific IgE. | A blood test is usually not the right test. |
What specific IgE tells you
Specific IgE measures antibodies against a single allergen — birch pollen, dust mites, peanut, milk. A positive result means sensitisation: the immune system recognises the allergen.
What the test does not tell you is whether you will have symptoms on contact. Some people have a positive IgE to a food they eat without any trouble. So a result without a history is not a diagnosis. An allergist reads it together with what happened to you, when, and after what.
A screening panel (for example total IgE with a basic set of allergens) makes sense as a first step. Use it when you do not know whether it is an allergy at all. With a clear history, a targeted panel or a referral to an allergist is more precise.
Ordering “everything” that exists does harm here. The more allergens you test in a person without symptoms, the more random positive results you get. Each one leads to needless avoidance of food.
Why IgG intolerance tests are out
Tests that measure IgG or IgG4 antibodies against dozens of foods are often advertised in Slovenia. They print a list of “unsuitable” foods. European allergy experts have repeatedly advised against them, with a clear explanation:
- IgG against food is normal. It forms because you eat the food. A high IgG to milk means you drink milk.
- There is no link to symptoms. The tests have not been shown to predict problems after eating.
- The consequence is harmful. Based on the result, people cut out ten or twenty foods. Their diet gets poorer, and the real diagnosis stays hidden.
That is why we do not recommend this test, even though many people ask about it.
Which test is right for which problem
| Suspected problem | The right test | Important |
|---|---|---|
| Allergy to pollen, dust mites, animal dander | Specific IgE or a skin prick test. | A result without a history is not a diagnosis. |
| Food allergy (immediate reaction) | Specific IgE for the suspected food. | After a serious reaction, go straight to an allergist, not to self-paid testing. |
| Lactose intolerance | Hydrogen breath test or a genetic test. | Blood IgG means nothing here. |
| Coeliac disease | tTG-IgA together with total IgA. | You must eat gluten until the sample is taken, otherwise the test is falsely negative. IgA deficiency is a common cause of a false negative result. |
| Gluten sensitivity without coeliac disease | There is no test. The diagnosis is made only once coeliac disease has been ruled out. | Rule out coeliac disease first, only then cut out gluten. |
| Histamine intolerance | There is no reliable blood test. The DAO enzyme in blood has not been proven useful. | Assessment is done with supervised elimination and reintroduction. |
| Bloating without a clear cause | First rule out coeliac disease and H. pylori infection, then a dietary work-up. | An allergy blood test rarely solves anything here. |
When not to test, but to see a doctor
- Swelling of the lips, tongue or throat, trouble breathing, dizziness after food or a sting. This is an emergency, not a question for the lab.
- Recurring hives lasting more than six weeks.
- Weight loss, blood in the stool, night-time diarrhoea. These are not signs of intolerance.
- A child who vomits after a certain food or stops growing. This belongs with a paediatrician.
Frequently asked questions
Is there a blood test for food intolerance?
For lactose there is a hydrogen breath test or a genetic test. For coeliac disease, tTG-IgA with total IgA. General IgG intolerance tests on dozens of foods are not diagnostic. European allergy experts advise against them.
Why does an IgG test flag so many foods?
IgG against food forms because you eat the food. A high IgG to milk means you drink milk, not that it harms you.
What does a positive specific IgE mean?
That you are sensitised to that allergen. It does not always mean you will have symptoms on contact. A doctor makes the diagnosis from your history and the result together.
Do I have to eat gluten before a coeliac test?
Yes. If you are not eating gluten, the test will be falsely negative. So get tested before you cut it out, not after.
Does it make sense to test many allergens at once?
Not in a person without symptoms. The more allergens you test, the more random positive results you get. Each one leads to needless avoidance.
Sources: EAACI — position that IgG and IgG4 food tests are not diagnostic for intolerance · EAACI — molecular allergy diagnostics and reading specific IgE · ESPGHAN/BSG — tTG-IgA with total IgA and the need to eat gluten before the test · ESPGHAN — hydrogen breath test in lactose intolerance · Maastricht VI — H. pylori in dyspepsia.
