M.B.
Apple juice is a more condensed form of apples and it is processed by the body very quickly since it has no fiber, so she's likely getting a spike in allergens that she can't process thus the rash. Fruit juices (and lots of fructose) in general can cause diarrhea, so these problems may be separable.
One very important thing to be aware of is that the skin testing is for immediate, IgE-mediated, allergies only. It will not detect any allergy that is delayed, even if the delay is 30 minutes. Those are IgG antibodies and few allergists seem to know about them. I learned all this the hard way after paying $600 out of pocket to see a top allergist and have him tell me that if my son had allergies to dairy and/or wheat, the symptoms would be overt.
However, we later learned (via IgA) testing that my son does have a dairy allergy and is gluten intolerant. The gluten intolerance can be the root for food allergies, as it has a tremendous negative impact on the immune system. Gluten is the protein found in wheat, rye, barley and most oats. The best test is this one:
http://www.enterolab.com/StaticPages/Frame_TestInfo.htm#s...
tinyurl: http://tinyurl.com/53dqbb
The IgA testing is looking for mucousal antibodies, which are found in the gut/stool. Thus, measuring these antibodies in the blood would not be nearly as helpful. IgG blood tests can be accurate for delayed food allergies. So, one could have either of these elevated and that shows a reaction to gluten or just have one elevated and still have that indicative of a reaction.
I realized what a bargain the enterolab test was after paying the $600 to an allergist that told us absolutely nothing new. My son also had intestinal malabsorption,based on the enterolab test which is why he had very slow growth. At two, the pedi said he would be petite. Since starting the GF diet, he has grown 4 inches and gained 20 lbs per year. That "petite" child is now 6'4" at 15.
I also have allergies to apples, but they are usually worse in the spring due to higher levels of birch pollen (they are in a similar family/group, thus cross react). So, you need to consider other foods in that family including almonds, peaches, apricots, nectarines, etc.
Also, frequent diarrhea is a hallmark symptom of celiac. But, the other end of the spectrum, constipation is as well. And, for all these kids on meds for reflux, that too is a symptom. Both my sons had reflux and my 2nd son had chronic constipation despite a good diet.
I found an excellent book at the library about hidden food allergies written by an allergy doctor that was clueful about all these things, because he was celiac himself.
http://tinyurl.com/3pz9fv
If your library doesn't have it, please ask them to order it. Hidden food allergies are the source of many of the problems that kids of mamasource mothers are often asking about - behavior problems, chronic infections, sleep problems, bedwetting, reflux, etc.