Cite this article as: Tan, J. Food allergy week #3 – diagnosing food allergy, Don't Forget the Bubbles, 2015. Available at: http://doi.org/10.31440/DFTB.6985
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?
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.
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.
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.
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.
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
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About Joyce Tan
Joyce Tan is a Paediatric SRMO from John Hunter Children's Hospital, currently on rotation at Gosford Hospital. She has a passion for medical volunteering and enjoys singing alligator goodbye songs with kiddies at her local church.
Author: Joyce TanJoyce Tan is a Paediatric SRMO from John Hunter Children's Hospital, currently on rotation at Gosford Hospital. She has a passion for medical volunteering and enjoys singing alligator goodbye songs with kiddies at her local church.
Food allergy week #3 – diagnosing food allergy
Tan, J. Food allergy week #3 – diagnosing food allergy, Don't Forget the Bubbles, 2015. Available at:
http://doi.org/10.31440/DFTB.6985
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:
Symptoms suggestive of IgE mediated or non-IgE mediated reactions:
The tests to identify IgE sensitisation to an allergen include:
What are the types of skin prick testing?
There are three types of skin prick testing used in allergy diagnosis.
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 allergen SPT uses positive and negative controls:
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:
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.
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 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
About Joyce Tan
View all posts by Joyce Tan