The amount of a C-1-tetrahydrofolate synthase, cytoplasmic when measured in blood serum. The trait is level of C-1-tetrahydrofolate synthase, cytoplasmic in blood serum (OBA_2042448, an Ontology of Biological Attributes term). Also known as: blood serum C-1-tetrahydrofolate synthase, cytoplasmic amount.