6-keto-Prostaglandin F1a (6-k-PGF1a) is a stable hydrolyzed product of unstable PGI2 (prostacyclin). PGI2 is derived from PGH2 which in turn is synthesized from the cyclooxygenase pathway. Through this pathway, it is the major product produced by the endothelial cells, although it is also synthesized in other cell types such as macrophages. Prostacyclin inhibits platelet aggregation and induces vasodilation. Quantitation of prostacyclin production can be made by determining the level of 6-k-PGF1a.