Skin Prick Testing (SPT) The how, why and what next…?

Skin prick testing is something that’s really important to diagnosing an allergy.

However it is also important to know that it is also just a part of the larger puzzle that is diagnosis.

So what you’re going to learn in today’s blog is that you can’t always take the number on the skin prick test and act on that number alone when it comes to feeding your child a food that they have shown an allergy to.

Skin prick testing (SPT) can sometimes appear to be done slightly differently depending on where you get it done, but essentially it is about getting a tiny amount of allergen just under the skin.

As well as SPT and sometimes a RAST blood test, it is essential that the doctor gets a good history.

If the child is already successfully eating a food, the specialist is unlikely to do a SPT on it because sometimes the test can still show a sensitivity and this can frighten the parent. In this case, sometimes a parent will stop giving that food because they have assumed the child is ‘allergic’ to it. Basically doing an SPT with a food that is being eaten is not recommended.

Children can show to be ‘sensitised’ to a food on SPT and if this food is then cut from the diet, there is more of a chance of the child becoming allergic to it because when it is reintroduced, the immune system is not used to it and mistakes it for an intruder!

SPT is performed by delivering a drop of the allergen onto an appropriate part of the body and agitating the skin so that that small drop can get just under the skin We normally use the forearm, back or on the leg. Often a grid is drawn so that each drop has its own space and we can label that space so we know which allergen is which

If the child has severe eczema, we need to find a part of the skin that isn’t too effected.

Sometimes SPT is done with a small metal lancet and other times it can be done with a single use plastic pricker that sits in a well of allergen. This single use pricker has 2 teeth that break the top layer of skin. It is then thrown away.

After the SPT has been performed, the child should be left for 15 minutes before the results are read. This is the optimum time to get an accurate result.

The results can vary from 0mm to as large as 30mm in some cases. The size of the hive allows the doctor to decide on a course of action going forward. Lots of decisions are made by the specialist when the hive is under 10mm, but over 10mm is classes as a moderate/large result and decisions would be very much on a case by case basis.

There are several things the doctor might suggest after an SPT.

• • Completely avoid that food

  • Book in for a food challenge

  • Trial the food at home (detailed instructions should be given)

  • Parent led challenge in hospital (depending on your provider)

Never trial a food at home that your child has had an allergic reaction to, unless your specialist has told you to and given you instructions.

At The Nest, we would recommend that when you have a food challenge, your child has had a recent SPT no more than 12 months previously. 6 Months would be even better!

Sometimes your child may have an SPT to a food that they have had a reaction to, and it shows up as a completely negative result. This can be because your child’s reaction was FPIES (Food protein-induced enterocolitis syndrome) this normally presents as profuse vomiting and lethargy. It is not an IgE mediated allergy so the SPT comes back as 0.

SPT should always be done with an immunologist/allergist that can interpret the results and organise appropriate treatment and follow up. We hear too often about places where parents can get SPT done but are then told to avoid all the foods and given no follow up or plan to re-do the SPT or reintroduce any foods.  This can be extremely detrimental to a child, and their family as well as family life when it comes to meals and socialising. It is also these kind of places that SPT to foods that a child is already eating and frighten a family into cutting them out of the diet.

When an SPT is done, there is a positive and negative control. So usually the first drop and prick is saline and the second drop and prick id histamine. This then shows us that the child’s body is acting normally if we see a 0 response to saline and a 3-6mm response to histamine. We then know that the other results are accurate. If we don’t see a histamine response, it may be that the child has had antihistamine which means the SPT will not be reliable.

We hope this has been helpful for you today!