Brayden is a 3 year old boy who has been referred to your clinic following one occasion where he developed some facial swelling and wheeze after ingesting peanuts. Brayden sounds like he may have a food allergy to peanuts. But how do we actually make the diagnosis?
This post is part of a series of posts on food allergy. The other post in series are: #1 – what is food allergy?; #2 – other types of food allergies; #4 – management of food allergy in infants.
Clinical history is paramount. Symptoms suggestive of IgE mediated reactions include:
- Acute urticaria, angioedema, persistent cough, severe vomiting soon after eating
- Contact urticaria to food
Symptoms suggestive of IgE mediated or non-IgE mediated reactions:
- Severe eczema unresponsive to standard therapies and treatment in children <2 years (co-existing food allergy rather than causal)
- Failure to thrive or poor nutrition not attributable to other causes in children <2 years
- Ongoing diarrhoea with or without blood loss in children <2 years
- Refractory vomiting in infancy
The tests to identify IgE sensitisation to an allergen include:
- Skin prick testing (SPT).
- Serum allergen specific IgE (a blood test formerly known as RAST).
- Medically supervised food allergen challenge if equivocal tests or doesn’t correlate with history.
- Patch testing and intradermal testing are not useful to confirm IgE mediated food allergy.
What are the types of skin prick testing?
There are three types of skin prick testing used in allergy diagnosis.
- SPT: main mode of testing for immediate IgE mediated allergy, low risks.
- Intradermal testing: IgE (drug allergy e.g. penicillins, venom, vaccines) and delayed-type hypersensitivity, not used for food allergy testing. Higher risks of adverse reactions, needs high levels of expertise.
- Patch testing: contact hypersensitivity, delayed-type hypersensitivity, not immediate/IgE-mediated allergy. Conducted by dermatologists and immunologists.
How does skin prick testing work?
Small amounts of allergens are introduced into the epidermis and non-vascular superficial dermis. This interacts with IgE bound to mast cells. Histamine and other mediators released. There is a wheal and flare reaction, peaking at 15 minutes.
Indications:
- Food reactions
- Allergic rhinitis/conjunctivitis/sinusitis
- Eczema
- Asthma
- Latex allergy
- EoE, eosinophilic GE, allergic bronchopulmonary aspergillosis
Food allergen SPT uses positive and negative controls:
- Volar surface of forearm or outer upper arm
- >5cm from wrist and 3 cm from antecubital fossa
- Drop then prick
- Read positive control at 10 minutes, allergen results at 15-20 minutes
- Measure mean diameter of wheal
- Observe patient for 20-40 minutes after the test
SPT increases the accuracy of diagnosis when added to the history and examination.
It differentiates allergic conditions and helps with allergen avoidance strategies, improved used of medications, and desensitisation treatment (allergen immunotherapy). It delivers safe, fast, results within half an hour.
However, it needs expertise to conduct and interpret correctly, especially in under 2 year olds. It can only be used on normal healthy skin (e.g. can’t be used in eczema flare – severe dermatographism can cause non-specific wheal and flare reactions to skin pricking alone). It also needs a co-operative subject.
Antihistamines and other interfering drugs (e.g. antidepressants, topical steroids reduce skin reactivity) should be stopped prior to SPT.
What are the indications for SPT?
SPT is not routinely indicated for:
- Non-specific rash without allergic features
- Chronic urticaria without allergic features
- Food intolerance without allergic features
- Chronic fatigue without allergic features
- Migraines/behavioural disorders
- Reactions to inhaled/resp irritants e.g. smoke/perfume
- Screening for allergy in the absence of symptoms e.g. family history
- LMW substances e.g. food additives
- Most occupational allergens
SPT for food allergens is valid but complex, and needs specialised practice.
Positive tests occur without clinical allergy, and negative tests can occur in the presence of clinical reactivity. There is a greater risk of anaphylaxis compared with aeroallergens. Commercial allergen extracts are available but non-standardised – sometimes it is better to use freshly prepared food extracts or the food itself.
How can we interpret the results?
It’s important when delivering post-test counselling that we explain the significance of the results.
- Positive SPT, not clinically reactive = clinically silent sensitisation
- Size of reaction does not correlate with severity of allergic manifestations
- Positive SPT does not predict the nature of allergic symptoms
- Positive SPT, clinically reactive but irrelevant as patient is not exposed to allergen and thus not cause of symptoms
- Positive SPT remains even when allergy has remitted e.g. pollen
- Negative SPT does not mean child will not develop allergy in the future
- Negative SPT can be due to insufficient representation of allergen in extract
- There are also non-IgE mediated reactions
- Food challenge may be indicated if SPT is equivocal or doesn’t correlate with the history
How should a food challenge be managed?
Pre-challenge:
Elimination diet, avoid food being challenged for at least two weeks. Note that prolonged elimination can result in acute severe reactions when challenged.
Atopic and chronic disease must be stable.
Avoid antihistamines or meds with anti-histamine properties, avoid for 5 half-lives of the agent.
Avoid steroid treatment for 7-14 days to prevent confounding results.
Avoid beta 2-agonist use as it may affect treatment if anaphylaxis occurs.
Perform challenges on empty stomach.
Challenge:
Preferred method for active and placebo challenges is on different days, but if on the same day, they need to be separated by at least three hours.
Start low, and can increase the dose 2-10 times each time, with at least 15-20 minutes between doses.
Administer increasing (fixed) doses.
The challenge is discontinued when the participant exhibits objective symptoms (e.g. vomiting, diarrhoea, and urticaria) at a specific dose or when the top dose is consumed without evidence of reactivity.
Results:
Clinical judgment of experienced investigator most important factor in calling result positive or negative.
Objective measures previously proposed:
- Exhaled nitric oxide (NO) measurement
- Facial thermography
- Serum histamine and tryptase level (concluded to be unhelpful in determining positivity of results)
Exhaled NO and facial thermography will be helpful as a immediate decisive tool to help standardisation but there are practical issues e.g. NO peaks 90 mins after challenge
