RAST IgE tests only show allergies, not intolerances. Can have false positives or negatives. And they're not very accurate for food allergies, from what I understand. Supposedly don't have to be actively eating the food for it to show up positive, but some find their IgE levels drop over time if they aren't exposed at least once in a while to the food. That said, my husband has high IgE to many foods, and it has been like this for him for a long time (seems his IgE count doesn't decrease over time so much)...so it may be different for everyone.
IgG ELISA blood tests show delayed reactivity (sensitivity reactions, some call them intolerances), but again, they can miss a lot or can have false positives. And some labs only look at IgG subclass 4 antibodies to foods, whereas there's another lab I was tested through that was considered positive if any of the 4 IgG subclasses showed a response (thought to be more accurate somehow...I just think of it as more comprehensive).
Caveate: patient must have eaten the foods tested within the previous few months to get any sort of accurate result, otherwise results may have false negatives.
ALCAT tests look at any immune response to foods (immedate or delayed allergy/intolerance) and are thought by some to be the most accurate. But like any test, it can have false positives or negatives. Thought to be the least objective of the 3, since it is based on lab tech interpretation of the interaction between food sample and blood cells. Caveate: patient must have eaten the foods tested within the previous few months to get any sort of accurate result, otherwise results may have false negatives.
The gold standard for food allergy is the double-blind, placebo controlled, oral food challenge (done in the doctor's office). I'm not sure anyone has come up with a gold standard for intolerances, but a detailed food diary in which every ingredient of everything consummed, plus a notation of any symptoms that change or appear, may reveal patterns that can then be tested with elmination/re-challenge diets.
And of course they have to make it confusing and use "intolerance" to refer both to delayed reactivity to food and enzyme deficiency related problems (such as Lactose Intolerance, which is caused by a deficiency of the enzyme lactase needed to digest milk sugar).
There are some problems with the tests, too: food test serums are not standardized, therefore you may get different results depending on which lab runs the test, the immune system has natural variations from day to day that may influence how strongly you react on a test to a food, which could potentially mean the difference between being told you do not have an allergy or intolerance to a particular food, when running the test on a different day may have caused the food to show up on the test as positive. Also, some MD's interpret any reaction on a test (whether it is class 1 or class 4) to be a positive result, others may only consider class 1 to be insignificant, class 2 to be equivocal (borderline), and only consider class 3 and up to be allergies / intolerances. Best suggestion I can give is to make sure and ask for an actual copy of the test results, that way you can see for yourself what the tests show and can ask your doctor more specific questions about the results.
I hope this helps - take care,
Gluten & Sulfite Sensitivity, Many Food/Inhalant/Medication Allergies & Intolerances, Asthma, Gut issues (dysmotility, non-specific inflammation), UCTD (Lupus?), Osteoporosis, Anemia, Lymphopenia, chronic malabsorption/malnutrition, Lyme Disease (Igenex Lab IgM WesternBlot positive; & as of 7/25/08, IgM is CDC Positive!), etc.; Currently TPN-dependent.
Meds: Zofran, Vitamin B12 Shots, Heparin (to flush PICC line).