Allergy Testing — IgE, Eosinophils & Food Intolerance

True IgE-mediated allergy: (Type I hypersensitivity) causes rapid symptoms — typically within minutes to two hours of exposure. Symptoms range from mild (sneezing, itching, hives) to severe (anaphylaxis, throat swelling, drop in blood pressure).

Food intolerance: (e.g., lactose intolerance) is different — it is not IgE-mediated and does not involve the immune system. However, IgG food sensitivity testing has become popular, though its clinical validity remains debated.

Total serum IgE: measures the overall level of IgE antibodies in blood, regardless of what they react to.

Normal range:: < 100 IU/mL (most healthy adults)

Elevated total IgE is seen in::

• Allergic conditions (asthma, eczema/atopic dermatitis, allergic rhinitis)

• Parasitic infections (very common in India — worm infestations cause marked IgE rise)

• Some immune deficiency states

• Certain skin conditions (Ommen syndrome)

Important:: Total IgE elevation alone doesn't identify what you're allergic to — specific IgE tests are needed for that.

ImmunoCAP (blood test):: The most reliable method. A blood sample is tested against a panel of allergen extracts. Results are reported as Class 0 (no sensitisation) to Class 6 (very high).

Common allergen panels tested::

• **Inhalant allergens:** House dust mite, cockroach, cat dander, Alternaria mould, Aspergillus, tree/grass/weed pollens

• **Food allergens:** Peanut, milk (casein/whey), egg (white/yolk), wheat, soy, shrimp, shellfish, fish

• **Latex, insect venom** (for occupational or anaphylaxis evaluation)

Eosinophil count (from CBC):: Elevated eosinophils (> 500/µL) in blood suggest allergic disease or parasitic infection. A simple CBC can raise the first flag.

Food allergy (IgE-mediated)::

• Rapid onset (minutes–2 hours)

• Can cause anaphylaxis

• Even tiny amounts trigger symptoms

• Diagnosed by specific IgE testing + oral food challenge

Food intolerance (non-IgE)::

• Slower onset (hours–days)

• Dose-dependent — larger amounts cause more symptoms

• Never life-threatening (though very uncomfortable)

• Examples: Lactose intolerance (enzyme deficiency), gluten sensitivity (non-coeliac), fructose malabsorption

Lactose intolerance:: Extremely common in Indians (~60–70% of adults). Caused by reduced lactase enzyme in adulthood. Diagnosed by hydrogen breath test or elimination diet — not IgE testing.

Coeliac disease:: Autoimmune reaction to gluten (not a true allergy). Blood markers: Anti-tTG IgA, Anti-Gliadin IgA. Confirmed by small bowel biopsy.

Consider allergy blood testing if you experience:

• Persistent runny nose, sneezing, or nasal congestion (allergic rhinitis)

• Chronic itchy eyes (allergic conjunctivitis)

• Eczema or recurrent urticaria (hives)

• Asthma — particularly if triggers are unclear

• Recurring digestive symptoms (bloating, diarrhoea) after meals

• Any previous episode of severe allergic reaction (anaphylaxis)

• Persistent eosinophilia on CBC without clear cause