Promising Diagnostic Tests

Technological advancement in tests, such as component-resolved diagnosis (CRD), make it possible to detect IgE sensitization in specific proteins or protein components instead of the whole food extract, which results in a low misclassification rate and a high sensitivity and specificity [1, 2]. The basophil activation test (BAT) has been used to effectively discriminate between sensitization to some allergens and clinical allergy [3, 8, 9]. However, further research is necessary before these tests can be used for routine diagnosis of FA [3, 8, 9].

Elimination Diets

Elimination diets cannot confirm FA diagnosis on their own, but can help to identify causative food in IgE and non-IgE-mediated FA [3]. These diets are also effective in reversing the clinical severity of mixed-type FA such as eosinophilic gastrointestinal disorders (EGIDs) [2]. The clinical history, allergy-focused diet history, and SPT or sIgE testing should guide which food should be eliminated [8]. There are several different types of elimination diets that can be used including the allergy-directed diet

Table 17.4 Characteristics of skin prick tests and serum-specific IgE tests for diagnosis of food allergy

Skin prick testing (SPT)

sIgE

Advantages
  • • Near instant results (within 15 min)
  • • High sensitivity (>90%)
  • • High negative predictive value accuracy, e.g., a negative SPT correctly indicates absence of IgE-mediated allergy in 90-95% of cases
  • • Can be performed on all age groups (even infants)
  • • Causes minimal discomfort
  • • Suspected food samples can be tested even if no commercial tests or food extracts are available
  • • No risk of anaphylaxis
  • • Order from physician’s office without specialist referral
  • • Can be used when SPT is contraindicated or ineffective:
  • - Pregnant women
  • - Significant anaphylaxis risk
  • - Severe skin disease (dermographism, extended or severe atopic dermatitis)
  • - Unable to stop using B-blockers or antihistamine for testing purposes

Disadvantages
  • • Moderately specific («50%)
  • • Moderate positive predictive value (50%), e.g., a positive results indicate sensitization but not allergy
  • • Cannot be used as a screening tool
  • • Cannot predict prognosis
  • • Cannot predict severity of future reactions
  • • Requires referral to specialist/allergy clinic

High chance of false positive results (e.g., the individual is not allergic)

Lower sensitivity (e.g., it may miss 10-25% of true allergies)

  • • Cannot predict prognosis
  • • Cannot predict severity of future reactions

Procedure (short summary)
  • • Apply small drop of commercially prepared food extract on skin of the forearm or upper back
  • • 3 cm between drops
  • • One drop with positive control (histamine) and one with negative control (physiological glycerine)
  • • Prick (1 mm) each drop with a new sterile lancet
  • • Hold for 3 s to avoid bleeding
  • • Remove allergen with blotting paper
  • • If the individual is sensitive to the food IgE antibodies, a wheal will develop on the skin.
  • • Measure wheal diameter after 15 min

Prick-to-prick method:

  • • To establish sensitization to:
    • - fresh food such as fruit and vegetables
    • - food for which commercial extracts are not available
  • • Method: prick the fresh fruit or vegetable and then the patient’s skin with the same lancet, followed by usual SPT protocol
  • • Phlebotomy required
  • • Analyses of the blood sample for food sIgE antibodies using standardized assays in certified laboratories.
  • - Fluorescence enzyme immunoassay (FEIA) tests such a ImmunoCap®
  • - Radioallergosorbent test (RAST®)

Results available depending on laboratory schedule, usually the next day

(continued)

Table 17.4 (continued)

Skin prick testing (SPT)

sIgE

Diagnosis of

sensitization to a food
  • • In general, a positive SPT result is defined by a wheal diameter >3 mm measured after 15 min
  • • Different wheal diameter cut-offs have been defined for egg, milk, and peanut FA in children [9]
  • • Food-specific diagnostic cut-offs for sIgE results where clinical symptoms are associated with a PPV >95% have been identified for several of the major food allergens
  • • A high total IgE level is often associated with multiple false-positive food sIgE levels and this is particularly important to consider in patients with severe eczema

Source: Refs. [1-3, 8, 9]

(elimination of specific food(s)), the oligo-antigenic diet, the extensively-hydrolyzed or amino acid- based elemental diet, the six-food group elimination diet, the four-food group elimination diet, the gluten-free diet, and the milk-elimination diet [2, 3]. In order to achieve symptom relief, the elimination diet should be followed for a period of 2-4 weeks if suspecting IgE-mediated FA, or for up to 6 weeks for non-IgE-mediated FA [8]. The elimination phase is then followed by a well-planned reintroduction phase of eliminated allergens according to specified guidelines [8].
 
Source
< Prev   CONTENTS   Source   Next >