Food Allergy: What You Need to Know

Diagnosis of Food Allergy

Clinical history is vital in the diagnosis of food allergy.^[10] A double-blind placebo-controlled food challenge is the gold standard in food allergy diagnosis but is used mostly in research. Open oral food challenges are used in the clinical setting, usually when it is believed that the individual is not allergic or has become tolerant.^[10] Skin tests and allergen-specific IgE levels have high sensitivity but moderate specificity.

Skin Testing

The positive predictive value (PPV) for skin testing is ≤ 50%, whereas the negative predictive value (NPV) is ≥ 95%.^[10]Because of a high rate of false positives, a positive skin test should be correlated with clinical history, whereas a negative skin test can be used to rule out an IgE-mediated allergy. The larger the diameter of a wheal in a positive skin test, the greater the probability of clinical reactivity; however, the size cannot predict the type or severity of a potential reaction.^[48,49,50] In one study, a median wheal size of ≥ 8 mm for milk, egg, or peanut in children younger than 2 years was found to be 95% predictive of food allergy.^[51]

Food-Specific IgE Testing

Food-specific IgE testing has similar specificity (~ 50%) but slightly lower sensitivity (> 90%) than skin testing.^[10] In vitro testing is useful when skin testing is not possible; for instance, if the patient is taking antihistamines, the patient has dermatographism, a large number of allergens need to be tested, the patient is acutely ill, or the patient has active atopic dermatitis and no skin is free of inflammation. Monitoring food-specific IgE levels over time is also useful, because falling levels could indicate that the patient is outgrowing the allergy. Like the size of skin tests, the concentration of food-specific IgE does not predict the type or severity of reaction, only the probability of clinical reactivity.^[48,49,50]

Using the ImmunoCAP system to measure food-specific IgE levels, studies have established PPV and NPV for common food allergens. These are frequently used to confirm an allergy diagnosis and to assess the risks and benefits of doing an oral food challenge. For example, an IgE level of 14 kU/L for peanut is > 95% predictive of clinical reactivity, and on the basis of this level, it would be unlikely that a patient would pass an oral food challenge.^[48] An undetectable IgE level (< 0.35 kU/L) for peanut is still associated with a 20% chance of reactivity.^[48]

The diagnostic levels of food-specific IgE for common allergens and associated PPVs for clinical reactivity are^{[15,52,53,54]}:

Egg: In children > 2 years of age, IgE level 7 kU/L (98% PPV); in infants ≤ 2 years, IgE level 2 kU/L (95% PPV)
Milk: IgE level 15 kU/L (95% PPV)
Peanut:IgE level 14 kU/L (95% PPV)
Fish: IgE level 20 kU/L (95% PPV)
Tree nuts: IgE level 15 kU/L (95% PPV)
Soy: IgE level 30 kU/L (73% PPV)
Wheat: IgE level 26 kU/L (74% PPV)

The probability of clinical reactivity on the basis of IgE levels depends on the particular food, and these levels are not comparable between foods. Therefore, the "classes" assigned to food-specific IgE levels by many laboratories are often confusing and may not be useful in predicting reactivity.

Overdiagnosis of Food Allergy

The high rate of false positives combined with the wide commercial availability of food-specific IgE testing (particularly the food allergy panels) has led to overdiagnosis and unnecessarily restrictive diets.^[10] Not only could this have a significant effect on quality of life, but it may be detrimental in children if dietary restriction affects nutrition and results in poor growth or development.

Food allergies can develop at any time; however, people uncommonly develop allergies to foods that are being regularly ingested. Cases of individuals who developed systemic reactions to food allergens that they previously tolerated after a period of strict avoidance because of atopic dermatitis have been reported.^[55,56,57] On the basis of these observations, it is not recommended that a food be taken out of a patient's diet as a result of a positive or even high IgE level, if the patient is tolerating the food on a regular basis. Evaluation by an allergist is recommended for patients who have atopic dermatitis but the causative food is not evident. Trial elimination diets can also be useful in certain situations but should not be continued if improvement is not seen.

Tests Not Recommended for Food Allergy Diagnosis

The following tests have not been supported by scientific data from controlled studies and are therefore not recommended by the American Academy of Allergy, Asthma, and Immunology to diagnose food allergies^[58]:

Applied kinesiology testing and Nambudripad's allergy elimination test;
Body chemical analysis;
Cytotoxic testing;
ELISA/ACT;
Electrodermal diagnosis;
IgG testing;
Provocation and neutralization; and
Pulse testing.

Some of these tests require ingestion or injection of the suspect allergen, which puts the patient at risk for a reaction. The European Academy of Allergy and Clinical Immunology (EAACI) published a strong statement against IgG and IgG4 testing for diagnosing food allergies.^[59] Several studies have presented evidence that food-specific IgG4 simply indicates repeated exposure to specific foods, and that IgG4 in conjunction with regulatory T cells seems to be an indicator of tolerance rather than hypersensitivity.^[59,60]

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