Steroids in prolonged use can effect the results, I doubt that one time before would but I can't be sure on that. Flagyl would not cause false negatives, if anything it would cause false positives since it is notoriously hard on the small intestine.
I am at a loss at this point, I am not sure what to make of results being so high than dropping, perhaps your daughter is an example of how inaccurate Gliadin AB's are. If your daughter had celiac and was eating gluten there is no reason for the antibodies to drop like that, active disease simply does not do that.
My only suggestion would be to retest using the methods I posted above as well as being off steroids for a week. Also did she get tested for IgA sufficiency? What led you guys to celiac to begin with?
In 2010 she was having constant bad stomach pains, missing lots of school. My husband has had horrible stomach problems all his life. we're always assumed that he had colitis. He has never been tested for it though, stubborn. So they tested her knowing my husband has some thing going on. She has been eating gluten all this time. I don't know what to think with those results. She will be retested next week. So I guess I'll just have to wait and see. thank you for your help.