The key differences
How food allergy is diagnosed
Diagnosis begins with a clinical history — symptom onset, timing relative to eating, severity and reproducibility. The most common diagnostic tests are skin prick tests (SPT) and specific IgE blood tests (previously called RAST or ImmunoCAP). A positive result means sensitisation — the immune system has produced IgE antibodies — but not necessarily clinical allergy. Many sensitised individuals tolerate the food in practice.
The gold-standard test for food allergy is an oral food challenge (OFC) — eating the food under controlled medical supervision. This is performed after initial testing when the results are ambiguous, or to assess whether a child has outgrown a diagnosed allergy.
Common food intolerances
Lactose intolerance — absence or reduction of the enzyme lactase — causes bloating, flatulence and diarrhoea after consuming dairy. It is confirmed by a hydrogen breath test or by lactase gene testing. Most people with lactose intolerance can tolerate small amounts (around 12 g of lactose — one glass of milk) without symptoms.
Non-coeliac gluten sensitivity (NCGS) is a diagnosis of exclusion: coeliac disease (autoimmune, confirmed by duodenal biopsy) and wheat allergy must both be ruled out first. A double-blind placebo-controlled food challenge is the only way to confirm NCGS, but is not routinely available.
The problem with self-diagnosis
Self-diagnosed food intolerances are common — studies suggest that while up to 20% of people believe they have a food allergy, clinical testing confirms it in only 2–4%. Unnecessary restriction of multiple foods can cause nutritional deficiencies, particularly in children, and significant lifestyle disruption without benefit.
Equally problematic: people who suspect only an intolerance when they have a genuine IgE-mediated allergy — and continue to consume small amounts of a food that could trigger anaphylaxis.