The amount of a Phosphatidylcholine (O-18:2_18:1) when measured in blood serum. The trait is level of Phosphatidylcholine (O-18:2_18:1) in blood serum (Ontology of Biological Attributes term OBA_2045133). Also known as: blood serum Phosphatidylcholine (O-18:2_18:1) amount.