The amount of a polypeptide N-acetylgalactosaminyltransferase 3 when measured in blood serum. The trait is level of polypeptide N-acetylgalactosaminyltransferase 3 in blood serum (OBA_2041665). Also known as: blood serum polypeptide N-acetylgalactosaminyltransferase 3 amount.