Why do we measure glutamic acid and not L-glutamine?

By the method used to run an amino acid assay, the amide group on glutamine is sheared off during the breaking of peptide bonds, changing the glutamine into glutamic acid. They are not the same thing…but by an amino acid assay, glutamine always shows up as glutamic acid. Unfortunately, it doesn’t help that the body cannot convert glutamic acid back to glutamine.