The gap between clinical allergy testing and commercial "sensitivity" testing is one of the largest evidence-to-marketing disconnects in consumer health.

Genuine allergy blood tests (IgE-based) are clinically validated, diagnostically useful, and well-integrated into the medical framework. IgG food panels are not. And the confusion between the two costs people money, restricts their diets unnecessarily, and in some cases, distracts from the actual investigation of their symptoms.

This article covers what allergy blood tests can and can't do, the IgG controversy, and how to navigate immune-related blood testing with evidence-based information.

A note before we get into it

Food allergy vs food intolerance vs food sensitivity

These three terms are used interchangeably in popular culture. They are not the same thing. The clinical distinctions are fundamental to understanding what blood tests can and can't detect.

Food allergy (IgE-mediated)

A true immune-mediated reaction. Your immune system produces IgE antibodies against specific food proteins. On subsequent exposure, these antibodies trigger mast cell degranulation, releasing histamine and other mediators that produce the classic allergic response: hives, swelling (angioedema), itching, respiratory symptoms (wheezing, throat tightening), gastrointestinal symptoms (vomiting, diarrhoea), and in severe cases, anaphylaxis.

Key characteristics: rapid onset (typically minutes to two hours after exposure), reproducible (occurs consistently on re-exposure), potentially severe (anaphylaxis), and detectable by blood tests (specific IgE) or skin prick testing.

Food intolerance

A non-immune-mediated adverse reaction to food. The most common example is lactose intolerance, the inability to digest lactose due to insufficient lactase enzyme. Others include fructose malabsorption, histamine intolerance, and reactions to food additives (sulfites, MSG, salicylates). Symptoms are typically gastrointestinal (bloating, gas, diarrhoea, abdominal pain) and dose-dependent.

Key characteristics: slower onset (hours to days), dose-dependent, not immune-mediated, not life-threatening, and not detectable by allergy blood tests (IgE or IgG). Diagnosis is typically clinical, through elimination diet and reintroduction under dietitian guidance, or specific tests like hydrogen breath testing for lactose or fructose malabsorption.

"Food sensitivity"

"Food sensitivity" is not a defined medical term. It has no agreed-upon clinical definition, diagnostic criteria, or pathological mechanism. The term is used commercially, primarily by companies selling IgG food panels, to describe a vague category of food-related symptoms that are neither true allergy nor defined intolerance.

The distinction matters because the test you need depends entirely on which category your symptoms fall into, and the commercial market deliberately blurs these lines.

IgE-mediated allergy: what blood tests can detect

Specific IgE (sIgE) blood testing (sometimes called RAST testing, though the original RAST methodology has been superseded by newer immunoassay platforms like ImmunoCAP) measures IgE antibodies directed against specific allergens in your blood.

What it can test for: Specific foods (peanut, tree nuts, egg, milk, wheat, soy, shellfish, fish, sesame, and many others), environmental allergens (dust mite, grass pollen, cat dander, mould), insect venoms (bee, wasp), and medications.

How it works: A blood sample is exposed to specific allergen proteins in the laboratory. If your blood contains IgE antibodies against that allergen, they bind, and the amount of binding is quantified. Results are reported as kU/L (kilounits per litre), with higher values generally indicating greater sensitisation.

What a positive result means: You are sensitised to that allergen. Your immune system has produced IgE antibodies against it. Sensitisation does not automatically mean clinical allergy. Some people are sensitised (positive blood test) but tolerate the food without symptoms. The blood test detects sensitisation; clinical allergy requires both sensitisation and symptoms.

What a negative result means: You are not sensitised to that allergen via the IgE pathway. This makes IgE-mediated allergy to that food unlikely, but doesn't rule out food intolerance or non-IgE-mediated food reactions, which operate through different mechanisms.

Sensitivity and specificity. IgE blood tests are highly sensitive (they rarely miss true sensitisation) but have moderate specificity (they can be positive without clinical allergy). Results must be interpreted in clinical context, ideally by an allergist who can integrate the blood result with the clinical history and, if needed, supervised food challenge (the gold standard for diagnosing food allergy).

When allergy blood tests are preferred over skin prick testing

Skin prick testing (SPT) is the traditional first-line investigation for IgE-mediated allergy. It's quick, inexpensive, and provides results within 15 to 20 minutes. So when does blood testing offer advantages?

When skin prick testing can't be performed safely

People with severe eczema or widespread skin conditions (insufficient clear skin for testing). People who can't stop antihistamines (antihistamines suppress skin prick reactions, producing false negatives, but they don't affect blood IgE levels). People with a history of severe anaphylaxis (skin prick testing carries a very small risk of systemic reaction). Very young children (practical challenges with skin testing in infants).

When multiple allergens need to be tested simultaneously

Blood tests can screen panels of allergens from a single blood draw, which can be more practical than multiple skin pricks in some clinical scenarios.

When quantitative results are needed

Specific IgE levels from blood tests provide a numerical value that can be tracked over time. This is useful for monitoring whether sensitisation is increasing, stable, or declining (relevant to outgrowing food allergies in children).

When access to an allergist is limited

Blood tests can be ordered by a GP and processed by any accredited pathology lab, whereas skin prick testing requires an allergist or trained clinic.

In general practice, SPT and specific IgE blood tests are considered complementary. Many allergists use both in combination. Neither alone is definitive. Clinical correlation is always required.

Eosinophils: the allergy marker hiding in your FBC

Most people who've had a Full Blood Count don't know that it contains an allergy-relevant marker.

Eosinophils are a type of white blood cell involved in allergic responses, parasitic infections, and certain inflammatory conditions. Your FBC reports the eosinophil count as part of the white blood cell differential.

Normal range: 0.0 to 0.5 x 10⁹/L (roughly 1 to 4% of total white blood cells)

What elevated eosinophils can indicate

Allergic conditions. Asthma, allergic rhinitis (hay fever), eczema, drug allergy. Mild eosinophilia (0.5 to 1.5) is common in allergic individuals.

Parasitic infections. Particularly helminth (worm) infections, relevant for travellers and people in endemic areas. Eosinophils are the primary immune defence against certain parasites.

Eosinophilic gastrointestinal disorders. Eosinophilic oesophagitis (EoE) is an increasingly recognised condition causing swallowing difficulties, food impaction, and reflux-like symptoms. It's diagnosed by endoscopic biopsy, but peripheral eosinophilia may provide a clue.

Drug reactions. Certain medications can cause eosinophilia, sometimes as part of a systemic drug reaction (DRESS syndrome).

Autoimmune and inflammatory conditions. Some autoimmune conditions (eosinophilic granulomatosis with polyangiitis, previously Churg-Strauss) are characterised by severe eosinophilia.

Haematological conditions. Rare but important: chronic eosinophilic leukaemia and hypereosinophilic syndrome.

If your eosinophils are mildly elevated on a routine FBC and you have hayfever, eczema, or asthma, that's a consistent finding. If eosinophils are significantly elevated (above 1.5) without an obvious allergic cause, your GP should investigate further.

The IgG food sensitivity test controversy

This is the section that prompted the entire article.

What IgG food panels test

These commercially available tests measure IgG (immunoglobulin G) antibodies against a panel of foods, typically 90 to 200 different items. Results are reported as colour-coded reactivity levels (low, moderate, high) for each food.

What IgG antibodies actually represent

IgG is the most abundant antibody class in human blood. IgG antibodies against food proteins are a normal physiological response to dietary exposure. When you eat a food, your immune system encounters its proteins and IgG antibodies are produced as part of normal immune recognition. Higher IgG levels against a particular food generally reflect more frequent or recent consumption of that food, not an adverse reaction to it.

What the evidence says

The Australasian Society of Clinical Immunology and Allergy (ASCIA) explicitly states: "IgG and IgG4 food antibody testing are unproven tests for diagnosis of food allergy or intolerance. These tests have no clinical relevance, are not recommended by any medical authority, and may lead to unnecessary dietary restrictions."

The American Academy of Allergy, Asthma and Immunology (AAAAI) and the European Academy of Allergy and Clinical Immunology (EAACI) concur. Multiple position statements and systematic reviews have concluded that IgG food testing does not diagnose food allergy, food intolerance, or food sensitivity, and that results should not be used to guide elimination diets.

Why the tests persist despite this consensus

Because they're profitable. Because the marketing is compelling ("discover your hidden food sensitivities"). Because the results always show something ("moderate reactivity" to 8 to 15 foods is typical). Because eliminating those foods sometimes coincidentally improves symptoms (placebo effect, or incidental removal of a genuine trigger that could have been identified through an evidence-based elimination diet without the test). And because the distinction between IgE (clinical) and IgG (normal physiology) is technical enough that most consumers don't know the difference.

The harm

Unnecessary dietary restriction. Nutritional inadequacy from eliminating staple foods. Anxiety around food. Distraction from evidence-based investigation of symptoms (could be IBS, coeliac disease, lactose intolerance, fructose malabsorption, all diagnosable through validated methods). Financial cost without clinical return.

IgG food panels are not clinically validated. They do not diagnose food allergy, food intolerance, or food sensitivity. They should not be used to guide elimination diets. If you have food-related symptoms, the evidence-based pathway is: GP assessment, coeliac antibody screening, dietitian-guided elimination diet, specific intolerance testing (hydrogen breath test), then allergist referral if true allergy is suspected.

Coeliac antibodies: the evidence-based "food reaction" blood test

If you're looking for a blood test that genuinely detects an adverse immune reaction to a food, coeliac disease is the answer.

Coeliac disease is an autoimmune condition triggered by gluten (a protein in wheat, barley, rye, and oats). The blood test, tissue transglutaminase IgA (tTG-IgA), detects antibodies produced in response to gluten exposure. It's clinically validated, highly sensitive and specific, and integrated into standard medical practice.

I covered coeliac testing in detail in the gut health article. The key facts: 1 in 70 Australians has coeliac disease, 80% are undiagnosed, and you must be eating gluten when you test.

Coeliac screening is where IgG food panels and evidence-based medicine diverge most sharply. An IgG panel might show "reactivity" to wheat, but it can't distinguish between the normal IgG response to dietary wheat and the pathological autoimmune response of coeliac disease. Coeliac antibody testing can. One costs $350 and provides no clinical information. The other costs a fraction and can change the course of someone's health for life.

Other immune markers worth knowing about

Total IgE. Measures overall IgE production, elevated in atopic individuals (those with a genetic predisposition to allergic conditions). A high total IgE supports an allergic phenotype but doesn't identify specific allergens. Useful as a screening tool alongside specific IgE testing.

IgA (total). Important in the coeliac context. Approximately 2 to 3% of people with coeliac disease have IgA deficiency, which can produce false-negative results on IgA-based coeliac testing. Total IgA should be measured alongside tTG-IgA to ensure the coeliac test is valid.

Complement levels (C3, C4). Components of the complement system, part of the innate immune response. Low complement levels can indicate complement consumption (as in active lupus or other immune complex-mediated conditions). These are specialist-ordered tests, not routine screening.

Immunoglobulin levels (IgG, IgA, IgM quantitative). Quantitative measurement of antibody classes, used to investigate immunodeficiency (recurrent infections, unusual infections) rather than allergy. Low levels may indicate primary or secondary immunodeficiency.

When to see an allergist vs when blood work is sufficient

See an allergist if:

Blood work through your GP or Bloody Good may be sufficient if:

How to prepare

For specific IgE allergy blood tests: No fasting required. No need to stop antihistamines (unlike skin prick testing). Can be tested at any time of day.

For coeliac antibodies: You must be eating gluten (2 to 4 slices of bread daily for at least 6 weeks). Going gluten-free before testing produces false negatives.

For FBC (eosinophils): No specific preparation. If eosinophils are elevated, your clinician may want to rule out parasitic infection, particularly if you've recently travelled to endemic regions.

For all tests: Mention all medications, supplements, and any recent illnesses. Corticosteroids can suppress eosinophils and IgE levels. Recent infection can transiently affect immune markers.

Tests to consider through Bloody Good

What to do after testing

If eosinophils are mildly elevated and you have known allergies: Consistent finding. No action needed beyond managing your allergic condition as usual.

If eosinophils are significantly elevated (above 1.5) without obvious cause: Your GP should investigate. Parasitic infection (stool microscopy, travel history), drug reaction (medication review), eosinophilic GI conditions (if swallowing difficulties or GI symptoms), or haematological causes (if very high).

If coeliac antibodies are positive: See your GP for gastroenterologist referral. Confirm with duodenal biopsy before starting a gluten-free diet. See the gut health article.

If coeliac antibodies are negative but food symptoms persist: Coeliac disease is ruled out (assuming you were eating gluten and IgA isn't deficient). Next steps: dietitian-guided elimination diet (low-FODMAP is first-line for IBS-type symptoms), hydrogen breath testing for lactose or fructose malabsorption, and clinical assessment for IBS. These are evidence-based pathways, not IgG panels.

If you've already done an IgG food panel and are restricting your diet based on the results: I'd encourage you to discuss this with your GP or a dietitian. The restrictions may be unnecessary and could be contributing to nutritional inadequacy or food anxiety. An evidence-based approach to your symptoms may be more effective and less restrictive than a colour-coded report based on a test that no major medical authority recommends.

If you suspect true food allergy (hives, swelling, breathing changes after specific foods): See your GP for specific IgE testing or referral to an allergist. True food allergy is a clinical diagnosis that requires validated testing and, in some cases, supervised food challenge. Don't self-manage suspected allergy. Anaphylaxis is life-threatening and requires an action plan.

Forty articles later

This is the fortieth and final article in a series that began with a ferritin of 28 and a man who'd never had comprehensive blood work done.

Over forty articles and approximately 190,000 words, I've written about every major blood test category that Bloody Good offers, and quite a few it doesn't. Iron, vitamin D, cholesterol, thyroid, PCOS, blood sugar, fertility, heart health, hair loss, weight gain, liver function, cortisol, B12, gut health, menopause, prostate, kidneys, sleep, brain fog, skin, biohacking, electrolytes, uric acid, calcium, selenium, Reverse T3, advanced hormone testing, inflammation, and now allergy and immunity.

The message has been consistent from Article #1 to Article #40: blood testing isn't about finding something wrong. It's about understanding where your body stands, and making decisions with data instead of assumptions.

Every article has ended the same way: get tested, save your results, compare over time, talk to your GP.

That advice hasn't changed. But if forty articles of evidence, stories, and practical guidance have made it more likely that you'll actually do it, then this series did its job.

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