The amount of a polypeptide N-acetylgalactosaminyltransferase 9 when measured in blood serum. The trait is Ontology of Biological Attributes term OBA_2041667 (level of polypeptide N-acetylgalactosaminyltransferase 9 in blood serum). Also known as: blood serum polypeptide N-acetylgalactosaminyltransferase 9 amount.