Case History - Section 3

Diagnostic testing

There are two main approaches to diagnostic testing in allergy:

• In-vivo skin prick testing (SPT). Food allergens eliciting a wheal of at least 3 mm greater than the negative control are considered positive. Overall predictive accuracy is < 50%; negative predictive accuracy is > 95% (negative skin prick test results essentially confirm the absence of IgE-mediated reactions).

Advantages of serum specific IgE:

• Antihistamines may interfere with skin tests hence serum specific IgE measurement may be advantageous where antihistamines cannot be discontinued for medical reasons
  
  
• Similarly, patients with extensive skin disease such as atopic eczema and those with dermatographia require serum specific IgE measurement
  
  
• Performance and interpretation of skin testing requires considerable skill. Serum specific IgE measurement can be useful in locations where skin testing is unavailable. Serum can be transported to laboratories performing the serum specific IgE measurement

Disadvantages of serum specific IgE:

• Many patients with eczema have very high levels of total IgE. False positive results may be seen in these cases. This is because there is so much IgE present in the blood sample that it shows up as a positive result for foods to which the person is not allergic
  
  
• A major disadvantage of measurements of serum specific IgE is cost

Investigations

Investigations performed when James first presented:

Immuno-assays for serum specific IgE antibodies against cow's milk and hen's egg protein showed concentrations of IgE antibody against:

• Cow's milk: 10.4 kU/L
  
  
• Hen's egg: 8.7 kU/L

A concentration of specific IgE = 0.35 kU/l is regarded as the cut-off level for a positive in-vitro test of specific IgE. While this may be appropriate for airborne allergens, it is a poor predictor of clinical reactivity to food allergens. Higher levels of specific IgE (decision points) for food allergens may correlate with clinical reactivity as evidenced by challenge testing. These levels are known as decision points. For further information access this paper.

Food-specific IgE concentration (kU/L) clinical decision points
	Egg	Milk	Peanut	Fish	Soya	Wheat
Reactive if equal to or greater than:	7	15	14	20	65	80

Skin prick tests could not be performed because of his severe atopic eczema.

Serum IgG antibodies were not tested because serum IgG antibodies to foods have no clinical validity in the diagnosis of food allergy, since most individuals make IgG antibodies to food they ingest: As a result the majority of individuals who have no clinical evidence of food allergy will have serum IgG antibodies to foods. The evidence for this is discussed in this review article.