Which is better for diagnosing allergies; skin or blood testing?

Allergic reactions are triggered if someone has been producing IgE antibodies which recognise a tiny protein allergen. When the person is exposed to enough allergen by eating, touching or breathing it in, the IgE antibodies may bind on its surface, cross-link making a bridge with allergen blood cells and trigger them to release allergy mediators like histamine, leukotriene and prostaglandins. These mediators lead to the swelling, inflammation, itching and possibly breathing difficulty or drowsiness that can be associated with an allergic reaction.

However, these IgE antibodies do not always lead to an allergic response;

  • the IgE antibodies need to be specific to that allergen
  • there needs to be enough specific IgE to pass a ‘tipping-point’
  • and there may be blocking antibodies or tolerogenic factors which prevent an allergic response

Both skin and blood tests aim to quantify the specific IgE which is termed ‘sensitisation’. However, low levels of sensitisation are commonly detected and are not necessarily high enough to cause an allergic reaction. Therefore, not all sensitised patients have an allergy. Non-specialists may wish to be ‘cautious’ and advise avoidance for all allergens with a slightly raised result, which can lead to unnecessary avoidance and needless stress.

Choosing if testing is a good idea

The first step is to decide if the problem may be caused by an allergy.

Immediate IgE-antibody driven allergic reactions are associated with fast-onset symptoms including;

  • Skin swelling; for instance eyelid, lip or tongue swelling
  • Immediate itching, often with ‘hives’ (raised swollen bumps surrounded by redness)
  • Vomiting
  • Voice changes
  • Hurting in the throat and coughing
  • Breathing difficulty, wheezing, rattily breathing and breathlessness
  • Drowsiness, paleness or sudden tiredness

The IgE allergic reactions most commonly start to resolve within one hour after allergen exposure has finished.

There are other types of ‘non-IgE’ allergic reaction which are not mediated by IgE antibodies. These tend to cause abdominal pain, diarrhoea, mucous or blood in the stool and may involve vomiting. These are more common in young children, who often out-grow these responses. Their diet may be broadened with careful planning.

Benefits of skin testing

Skin prick testing involves placing a drop of the food or environmental allergen on the skin, and introducing this allergen to the immune system by touching it with a lancet into the skin.

If the food allergen in question in not carried in our panel of extracts, parents can bring an unusual food for modified skin testing, where we use a tiny cube amount of food to prime the lancet.

The child only feels a tiny mark, and all the tests are over within minutes. In order to be ready for this testing, children must avoid taking antihistamine (cetirizine ‘Piriteze’, levocetirizine, loratidine, desloratidine, fexofenadine) for one week (5 days) before the testing. Piriton (also called Allerief or chlorphenamine) should be avoided for at least 48 hours. If antihistamine is taken closer to the skin test, there is a risk that hives may not arise when there is still a risk of reaction to the allergens.

The benefits of skin prick testing include;

  • Weal development within 15 minutes to demonstrate sensitisation
  • Easy treatment with antihistamine afterwards if child is uncomfortable
  • Clear demonstration of a response in real time to family and team
  • A chance to discuss the appearance of hives for the child’s own skin

Sometimes blood testing is preferable, for instance;

  • If antihistamines have been taken shortly before testing
  • If a food allergen is seasonal or unavailable for skin prick testing
  • If specialist component blood testing can discriminate sensitisation to particularly risky nut proteins
  • If pollen allergy is complicating a nut allergy and may be masking a true primary nut allergy

Micro-array IgE blood tests allow over 100 allergens to be tested with a few milliliters of blood. These are useful for;

  • Home testing where the child is over 6 years and does not want to attend a phlebotomy
  • Screening for a wide range of potentially unknown allergen sources
  • Testing to confirm other allergen tests

The clinician team undertaking the test is the professional who invoices for payment. For example, if Dr Marrs recommends sending out and completing a home-test kit, or if he undertakes skin prick testing then this is invoiced through Proactive Allergy. However, the hospital clinic bills for phlebotomy sampling and the blood test processing, and these are billed separately. There is therefore no benefit for clinical staff in recommending extraneous blood tests which do not help assure your child’s safety.

What is a Food Challenge?

In the unusual circumstances that skin and or blood testing do not show a clear result, then the final best-evidence approach is to undertake food challenge testing where safe to do so.

In this circumstance, your child would be brought to one of our clinics when in good health, with a single portion of a food that the child is willing to eat. After a clinical review, possibly repeat skin prick test and signing consent, your child will be offered to eat the food in one single portion, or as an incremental challenge in increasing amounts, until a full portion has been tolerated or the early signs of a reaction have been detected. This is the gold-standard proof of allergy in the research setting, and can be arranged where clinically relevant to do so.

Choosing the best test(s) for your family

A wide range of considerations go into choosing the best tests for your child. In the early stages, it is important to consider whether you wish to undertake the first appointment as a remote consultation. It is important to consider which clinic you are choosing and whether you wish your child(ren) to take time out of their day. Dr Marrs is happy to discuss the different options with you according to your availability. Our goal is to enable safest best diagnosis around your home, work and preferential circumstances.

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