Normal value is considered, I believe, between 0-3 for Transglutaminase. Over 30-40 is almost always indicative of Celiac Disease. My level was over 100. Below is a list of the serologic tests for celiac disease. My GI ran all of them on myself, as well as all of my first degree relatives (this disease runs in families). My young daughter also was found to have Celiac and was asymptomatic.

tissue transglutaminase (tTG) immunoglobulin A (IgA) and tTG immunoglobulin G (IgG)
endomysial antibody (EMA) -IgA
deamidated gliadin peptide (DGP) -IgA and DGP-IgG
If you were tested while you were consuming at least the equivalent of one slice of bread (made with wheat flour) per day for three months, and your values were negative, than chances are good that you don't have a problem with gluten. If you were tested while not consuming gluten, you could have a false negative. Unfortunately, gluten has to be in the body's system for an accurate test result. If you're looking for more information on Celiac and gluten intolerance, I highly recommend Dr. Peter Green's book, "Celiac Disease: A Hidden Epidemic". It is "hidden" because it is thought that roughly 50% of people who have this disease are undiagnosed or misdiagnosed. Many people are completely asymptomatic. Others, like myself have very strange and seemingly unrelated symptoms. I hope this helps! 🙂