How to test for food intolerance, and what an IgG result can and can’t tell you

Food allergy, intolerance or “sensitivity”?

These words get used as if they mean the same thing, and they don’t. The difference decides who you should see and which kind of test, if any, helps. Healthdirect, the Better Health Channel and ASCIA draw the line like this:

How food intolerance is usually worked out

Healthdirect is plain about it: there are no specific tests for food intolerance. What works is slower and more personal:

1. Write it down. What you ate, how much, and what you noticed over the next day or so. Patterns that feel random often aren’t once they are on paper.
2. Take the suspected food out for a set time. One food or one group, for a short, defined period, rather than a long list for as long as it takes.
3. Bring foods back one by one. Reintroduce them on purpose and watch. Because intolerance usually depends on the amount, you may find a smaller serve is fine.

ASCIA calls a temporary elimination diet, supervised by a dietitian and a medical practitioner, the only reliable way to tell whether diet is playing a role. It also warns that restricted diets kept up for a long time can cause problems with nutrition, especially in children, and Healthdirect notes some restrictive diets have been linked with eating disorders. A trial should have an end date.

What an IgG food test measures

An IgG food test uses a small blood sample, often a dried blood spot from a finger prick, and measures IgG antibodies against proteins from a panel of foods. On the sample report from the lab we use, each food gets a reaction class from very low to very high, set against a reference range, so the report reads as a list of foods from low to high.

IgG is a normal antibody. ASCIA notes IgG to foods is commonly found in healthy adults and children, with or without symptoms, and that it reflects exposure to a food rather than disease. The European Academy of Allergy and Clinical Immunology goes further and describes it as a sign of repeated exposure, and as an indicator of tolerance rather than of a reaction.

What the evidence says about IgG testing

The main allergy societies are unusually direct. ASCIA’s position paper rates IgG food antibody testing as an inaccurate test and finds no credible evidence that it is useful for identifying food allergy or intolerance. Its patient information warns that tests like this can lead to costly and often dangerous dietary restrictions. The European and Canadian allergy societies reached the same view, the Canadian statement noting that positive IgG results are to be expected in healthy adults and children.

There is some research on the other side. In a 2004 randomised trial of 150 people with irritable bowel syndrome, an IgG-guided elimination diet reduced symptom scores by 10% more than a sham diet over 12 weeks, and the authors called the approach worthy of further research. The antibody testing, and the diet sheets, came from a company that sells IgG tests. It is one trial in one condition, and it doesn’t show that an IgG test can identify intolerance.

So the honest place for an IgG test is narrow. It can’t tell you that you are intolerant to a food. At most, it can suggest which foods to trial first, and the trial is what gives the answer.

When a test is the wrong first step

Food isn’t always the question. It helps to know which question is yours:

If you do an IgG test, how to use the result

Read the report as a list of questions, not a list of rules. Pick the one or two foods that line up with what you have actually noticed, trial them for a set time, and bring them back. Leave the rest of the high results alone unless they give you a reason not to.

And keep eating as widely as you can. The goal is to find what really bothers you, and to end up with more on your plate than you started with, not less.

If you have decided an IgG test is a useful starting point, our Food Intolerance Test page has the options and what each one includes.