Skin vs. Blood Testing for Allergies

Many people have asked questions about the process of W's allergy testing and the difference between the skin and blood test, so I thought I would share a bit about it.  

Skin Tests

An allergy skin test involves pricking the skin with a trace amount of an allergen and then watching how the skin reacts. The nurse or technician administering the test marks the skin on his back so that they can track which reaction is the result of which prick.  With each of W's skin tests, a positive control (typically a histamine solution) and a negative control (saline usually) have both been administered along with the other pricks.  If I remember correctly, the far right two pricks were the negative (middle) and positive (bottom) in the below picture.  After all of the pricks are made, we wait and watch as hives form (or hopefully don't) on those spots.

I challenge all of you to try to hold a squirmy 3/5/9/ or 11 month old in your lap who has just gotten between 4 and 12 shots in their back, all the while not touching said shots and not letting them touch/itch/rub them either. Oh, and go ahead and do this for 20 minutes. Fun!

W's skin test at age 9 months

After the allotted time has elapsed (shout for joy) the nurse will come in and measure each of the spots using a series of numbers and words that can only be described as a foreign language: wheal, flare, pseudopod and satellites.

Apparently these numbers and words all mean something and correlate into a final ranking.  If your ranking is above a certain number (based on age) then you are considered to have a certain percentage chance of having an allergic reaction (see an example of W's results below.)  These numbers are measures of IgE antibodies released by a persons body in response to that allergen.

W's has had 4 skin tests during the first year of his life, the first at 3 and 1/2 months old.

Blood Testing
Between two different allergist and a few pediatricians, there have been conflicting recommendations on the best course of action to test for allergies. Some say blood tests are inaccurate before a certain age and some said the same for skin tests.  Because W's first reactions occurred before 6 months of age, the consensus was that skin testing was more accurate than blood testing.   Since then, W has had two blood tests  One at 7 months and one at 11 months old. For the most part, blood testing seemed to be ordered to either confirm a skin test allergy or to have a base IgE to compare against. 

Blood testing is just as it sounds, whereas a lab draws vials of blood and sends them off for testing. Unlike the immediate gratification, or lack there of, with skin testing, results take a few days to come back.

Results Example

Because eggs were the first know food allergy for us, it is the one for which W has been tested the most number of times. His first test (skin) at 3 and 1/2 months revealed an IgE level of 5. Children under the age of 2 years are considered to have a 100% change of reaction to eggs at a level 2. The rest of his testing for eggs are as follows:

7 months (blood): IgE level of 14

9 months (skin): IgE level of 30

Eggs will likely be tested again in the coming months to see if his levels continue to rise.

My Little Trooper

I have to say that my sweet boy is such a trooper. Counting only pricks and shots for allergy testing, W has been poked and prodded more than 30 times. While he does cry (I'd be more freaked out if he didn't) he recovers and is back to his sweet and happy self in no time.

Disclaimer: I dropped out of the pre-med program in my sophomore year of college, so please forgive my lack of medical knowledge and any scientific inaccuracies. I am writing according to my understanding and what memory remains after having three children.