Enterolab has a high positive diagnostic rate... I think something like 60-70% of those testing turn up positive (keeping in mind this is already a symptomatic population and a sensitive method of testing). They gets positives on something like 30-40% of the general "healthy" population.

Do you know if you have ever been tested for Total IgA (by blood test)??? Some people have a condition called "Selective IgA deficiency", and they can fall through the cracks in both blood testing and stool testing. I have no way of knowing, but this might apply to you. If you read the second article below, you may get a sense for whether this sounds like you or not. People with IgA deficiency tend to get sick more often with respiratory tract and urinary tract infections requiring antibiotics, often have food sensitivity and autoimmune disease.

From what I remember, Enterolab says they can have some false negatives due to IgA deficiency, as their tests look for IgA class antibodies. If you look through the FAQs on their website you will probably find this mentioned.

Dr. Fine suggests a gluten free diet trial in the event of negative tests for this reason. If you respond positively to a gluten free diet... then you have your answer... even though you may wish you had something on paper to point to. Give a gluten free diet a very honest try, strictly gf, for at least 3-6 months... and listen to what your body tells you.

Here are two papers on IgA deficiency. If you are IgA deficient, or below range in total IgA, this might explain your predicament. You could ask to be tested for this to rule it in or out.

Celiac Disease and Immunoglobulin A Deficiency... by Kumar, et al.

Selective IgA Deficiency -IDF Patient/ Family Handbook

Hope this helps!
Cara