Gag’s Urine Mucopolysaccharides are a group of mucopolysaccharide components excreted in the urine, which can indicate metabolic disorders related to lysosomal storage diseases. Gag’s urine tests are particular biochemical assays done through quantitative assays or qualitative spot screening tests. Tests help healthcare professionals evaluate the deficiency of enzymes needed to break down glycosaminoglycans or mucopolysaccharides, which cause mucopolysaccharidoses.