The amount of a Phosphatidylcholine (O-18:0_20:4) when measured in blood serum. The trait is Ontology of Biological Attributes term OBA_2045127 (level of Phosphatidylcholine (O-18:0_20:4) in blood serum). Also known as: blood serum Phosphatidylcholine (O-18:0_20:4) amount.