Skin prick tests

Skin-prick allergy tests for foods are extremely useful.  They are also safe, painless and simple to do.  This is a precise way to identify the offending foods.

Eczema cannot be properly managed without discovering the foods and inhalants that are the culprits setting off the eczema cycle.  Skin-prick testing identifies possible allergens.  When done properly, it is a gentle test. 

How is it done?

Skin-prick tests are gentle.
See this YouTube video: Skin Prick Tests Do Not Hurt so that you can see exactly how we do skin testing:

Skin-prick testing is simple and should be pain free.  A small droplet of oil, which contains the specific allergen (a food or an environmental-allergen) is placed on the skin.  This is usually done on the back, for babies and toddlers, and on the forearm for older children and adults.

Next, the skin surface is very gently pricked through this droplet.  A positive test starts to occur within 5–10 minutes: a small wheal (like a little mosquito bite) develops.  The size of the reaction (the wheal size) is measured at 10–15 minutes.  It can get a little itchy but settles down in about half an hour.  In children, skin prick testing is accurate.  With age, the skin-prick tests tell you more about past food allergies rather than present clinical intolerances.  Therefore in adults, skin tests for foods are less useful.

How does it work?

The skin-prick test measures immediate reactions.  It is a test that measures the “IgE-sensitivity” to the allergens which have been pricked gently into the skin.  As soon as the allergen gets into the skin, it comes in contact with the allergy cells in your skin (the mast cells, which are packed full of histamine).

If your mast cells have been sensitized to that allergen, they will immediately release their histamine, and this creates the wheal, just like a mosquito bite.  

The skin-prick test is a biological reaction and takes about 10–15 minutes to get to its maximum intensity. 

If the child has been treated with an antihistamine within the preceding 24 hours, then the skin tests will be attenuated. In other words reactions  will be less dramatic and will take longer to show up.

Skin-prick tests accurately identify food allergens

Who should get tested?

We recommend that all babies, all children and all adults with eczema or asthma should be tested with a selection of skin-test allergens.  All breastfed babies with eczema need skin-prick testing.  All people who have troublesome allergy symptoms should be investigated by skin-prick tests. Consequently, in my clinic, I do skin-prick testing for all children with eczema.

Skin-prick tests can be done at any age: even on new-born babies (although it is less reliable at this age).  It is my routine practice to do the first set of skin-prick tests at about three months of age.  By this age the baby has developed the capacity to mount a specific IgE-response in the skin.  But, if the baby has bad eczema in the first few weeks of life, then it is useful to do the skin tests earlier.

This is especially helpful in breastfed babies with eczema.  Most of the time these babies are reacting to food proteins that are coming through in their mother’s breast milk.  By accurately identifying what the baby’s skin is reacting to in the breast milk, we can then advise the mother about what foods that she should be avoiding in her diet.  Usually, the eczema will completely go away.

For instance, Thomas’s mum said:

“Knowing what foods to avoid, by the skin tests, made the world of difference.  As soon as I cut eggs out of my diet (I was breastfeeding) his skin cleared up.”

What allergens can be tested?

There are two categories of allergens that you can be tested for: food and inhalants.

Foods: The most common and useful foods to test for are:

cow’s milk
egg white
peanut
fish
tree nuts
wheat
soy
cashew.

In adults, testing for shellfish is important. Usually, food reactions to fruits and vegetables cannot be successfully tested by skin tests – that is because these fruit and vegetables reactions are usually caused by the chemicals in these foods, rather than by the food proteins.

Inhalants: The important inhalant allergens (also called aero-allergens) are:

house dust mite
grass pollens (rye grass, timothy grass)
tree pollens (especially birch)
animal dander (cat fur, dog hair, horse hair, and feathers).

Who can do skin tests?

Most allergists do their own skin-prick testing.  We carry out skin-tests on my patients as a routine part of their normal consultation.  This gives me the opportunity to interpret their results as soon as they come up, and in context with their clinical story. They are so useful because they can give immediate answers to the eczema triggers.

Also, many medical laboratories are set up to perform skin-prick tests.  You can ask your doctor to refer you to one of these laboratories for these tests.  Yes, you should ask for skin-prick tests to help identify any suspected allergens if you have eczema, asthma, hay-fever and suspected food allergy.  Many doctors do not understand the value of these skin-prick tests so they need to be encouraged to request these tests.

If you are not able to get these skin tests done, then another option is to get the IgE blood tests: these are called EAST (enzyme allergosorbent test) or RAST (radio allergosorbent test) tests.  These tests give you the same information as the skin-tests.  They test for levels of “specific IgE antibodies” in the blood to each of the allergens of interest.

When should skin tests be repeated?

It is good news that most food allergies go away as you get older.  Coinciding with this, the skin-prick tests also wane with age.  That means carrying out regular skin-prick testing is useful in tracking the current status of the food allergy.  Also, repeating the skin-prick tests help to confirm to the parents that they are indeed making a difference by strictly controlling their child’s diet.

It is our practice to conduct skin-prick tests at the ages of 3–6 months, 12 months, 2 years, 3 years, 5 years, 8 years and 12 years.  We find that giving families accurate feed-back about the progress of their allergies is extremely helpful.

Monika’s story:

A concerned mum, Monika, wrote to me about her baby boy.  I am told similar stories most days. She says:

“I would really appreciate your advice.  My son is 6 months old, and has suffered from flare-ups of his eczema since very young (as well as cradle cap).  He has been only had breast milk up to now.  His eczema has moved from covering his back, to his tummy and now to his legs (it is worst behind his knees).  It is now beginning on his face.”

“We have been very careful with any contact (no laundry liquid, all cotton clothing etc).  I have tried a wide variety of emollients on him with no improvement, other than hydrating the skin for an hour or two!  Reluctantly, I am now trying a steroid (hydrocortisone) cream.  I have heard that ‘allergy parents’ may be the reason for infantile eczema.  I have mild asthma and hay-fever and my husband has a very small patch of eczema himself.  Would this be why our son has eczema?  Or could this be a food allergy coming through my breast milk?”

“What action (blood tests/ skin-prick tests etc?) would you recommend?  We can’t just sit and watch his eczema get worse.  Our doctors have just generalised the condition – hoping that he may just grow out of it.  Many thanks in advance, Monika.”

My reply was: “Yes I can help.  I suggest the following:

1.  Get skin prick tests if you can – especially to egg, milk and peanut.
2.  Start a probiotic.
3.  Whilst breastfeeding you will probably need to go off dairy, egg and peanut.
4.  Yes, use a 1% hydrocortisone cream for another month.
5.  Avoid all egg, dairy and peanut in his solids.

I hope that this helps.  With appropriate management I expect nearly all babies to get completely better.”

Monika wrote back a few days later:

“I’m so glad that you pointed me in the right direction.  The doctors who I saw took a lot of convincing – or should I say begging – to get the skin pricks and bloods done.  I could see my diet was already making a difference before the tests so I was convinced he was allergic to some of the foods that I had been eating.”

A few weeks later she wrote again:

“I am just following up on our earlier email exchange.  Our baby son had the skin-prick tests and blood tests.  This showed he had allergies to dairy and peanut.  Since avoiding these foods in my diet, my breastfed son’s eczema is 95% resolved.  Just thought you’d like to hear that your advice was spot on.  And the relief we feel is unexplainable!  Many thanks.  Best Regards, Monika.”

This baby is typical.  His eczema would have gotten progressively worse if mum had not changed her diet.  The skin test helped mother work out what foods she needed to avoid.  Because the skin tests gave her certainty about her diet, it was easier for her to comply.

Skin prick tests are not the whole answer

But skin tests are not the whole answer.  This is because many foods, especially dairy and gluten can cause the delayed onset type of reaction.  Under these circumstances, the skin tests are negative for dairy and gluten.

If the skin-prick tests are negative, then dairy or gluten should still be suspected.  To make a diagnosis of gluten-sensitivity, blood tests are needed.  Should the eczema not settle – again blood tests are needed to check out immune function.  Food reactions are not the only cause of eczema.

 

This is an excerpt from Dr RodneyFord’s book: “Eczema! Cure it!”  – available as an eBook

 

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