IgG "food sensitivity" tests measure exposure to food, not allergy to it

Blood tests sold direct to consumers as "food sensitivity" or "food intolerance" panels usually measure immunoglobulin G4 antibodies against dozens or hundreds of foods. A European Academy of Allergy and Clinical Immunology task force examined that practice and concluded that food-specific IgG4 does not indicate food allergy or intolerance, but rather a physiological response of the immune system after exposure to food components — and that testing for it should not be performed in cases of food-related complaints [s1]. The antibody the test detects is, on the task force's reading, a marker of tolerance rather than of hypersensitivity [s1].

That verdict is nearly two decades old. The tests are more widely sold now than they were then.

What the task force actually said

The report's reasoning has three parts, and each is a separate objection.

First, the tests produce positives without symptoms. Many serum samples show positive IgG4 results with no corresponding clinical symptoms [s1]. A test that flags foods a person eats without difficulty is not measuring the thing the customer bought it to measure.

Second, there is no mechanism. The task force notes the lack of convincing evidence for histamine-releasing properties of IgG4 in humans, which removes the biological basis for treating it as an effector of food hypersensitivity [s1].

Third, there is no diagnostic-accuracy evidence. The report states there are no controlled studies on the diagnostic value of IgG4 testing in food allergy [s1] — meaning the tests have never been shown, against a reference standard, to identify anything.

Against that, the task force sets out what IgG4 against food does indicate: that the body has been repeatedly exposed to that food and recognised its components as foreign proteins, a process the report links to the activity of regulatory T cells [s1]. Read that way, a long list of "reactive" foods on a panel is a list of things the person eats often.

The task force also notes what drives the market: many people believe their symptoms are related to food without diagnostic confirmation of a causal relationship, and screening panels covering hundreds of food items are sold to young children, adolescents and adults alike [s1].

Allergy, intolerance and "sensitivity" are three different words

Food allergy in the clinical sense means a reaction mediated by immunoglobulin E, a different antibody class from IgG4; the US adult prevalence survey classified reports as convincing only when the described symptoms were consistent with an IgE-mediated reaction [s2]. Food intolerance is a separate problem, and the EAACI report treats it as separate too: it concludes that IgG4 testing is irrelevant to the laboratory work-up of food allergy and of food intolerance alike [s1]. "Food sensitivity" is not a category either document uses.

The distinction is not pedantry, because the consequences differ. An IgE-mediated allergy can be dangerous and warrants avoidance. An intolerance is a dose and comfort question. A test result with no diagnostic meaning, used to justify eliminating a dozen foods, produces the costs of avoidance with none of the benefit.

How big the gap between belief and diagnosis is

A survey of more than 40,000 US adults, administered by internet and telephone from 9 October 2015 to 18 September 2016 and weighted to produce nationally representative estimates, asked about food allergies and about the symptoms people experienced [s2]. Reports were classified as convincing when the described symptoms to a specific allergen were consistent with an IgE-mediated reaction [s2].

Estimated convincing food allergy prevalence among US adults was 10.8% (95% CI 10.4-11.1), while 19.0% (18.5-19.5) self-reported a food allergy [s2]. The most common convincing allergies were shellfish (2.9%), milk (1.9%), peanut (1.8%), tree nut (1.2%) and fin fish (0.9%) [s2].

The gap between those two figures is the space these tests are sold into. The survey's authors draw the conclusion directly: it is crucial that adults with suspected food allergy receive appropriate confirmatory testing and counselling, so that food is not unnecessarily avoided and quality of life is not unduly impaired [s2].

That is not a dismissal of the symptoms. Among adults with a convincing food allergy, 51.1% had experienced a severe reaction, 45.3% were allergic to multiple foods, and 48.0% developed their allergies as adults [s2]. Only 24.0% reported a current epinephrine prescription, while 38.3% reported at least one lifetime emergency department visit related to food allergy [s2]. The problem is real and, on those figures, under-treated at the same time as it is over-claimed.

The same pattern in children

A weighted survey of 38,480 US children, conducted from June 2009 to February 2010, estimated childhood food allergy prevalence at 8.0% (95% CI 7.6-8.3) [s3]. Among food-allergic children, 38.7% had a history of severe reactions and 30.4% had multiple food allergies [s3]. The most common allergens were peanut (25.2% of food-allergic children), milk (21.1%) and shellfish (17.2%) [s3]. The study also reported disparities in clinical diagnosis by race and income [s3].

Both surveys rely on self-reported symptoms rather than oral food challenges, which is the reference standard, so both are estimates with a defined weakness: they can classify a report as convincing without confirming it [s2] [s3]. That limitation cuts in the direction of caution about all self-reported prevalence figures, including the ones quoted by companies selling tests.

What follows

The honest position on a commercial IgG or IgG4 food panel is not that it is inaccurate. It is that the analyte it measures has no established relationship to the symptoms it is sold to explain, and that a specialist body examined the question and recommended against the practice [s1]. Someone with persistent food-related symptoms has a real problem worth investigating; the argument here is about which investigation gives an answer.

This article describes published evidence and a professional-society position. It is not medical advice, and decisions about testing or elimination diets belong with a clinician.

Sources

• Testing for IgG4 against foods is not recommended as a diagnostic tool: EAACI Task Force Report — Allergy, 2008-05-16
• Prevalence and Severity of Food Allergies Among US Adults — JAMA Network Open, 2019-01-04
• The prevalence, severity, and distribution of childhood food allergy in the United States — Pediatrics, 2011-06-21