Prostaglandin I (Prostaglandin I2; PGI2) is a potent vasodilator and inhibitor of platelet aggregation that is formed from arachidonic acid primarily in the vascular endothelium and renal cortex by sequential activities of cyclooxygenase (COX) and PGI2 synthase. It is non-enzymatically hydrated to 6-keto PGF1α (t1/2 = 2-3 minutes), and then quickly converted to numerous downstream metabolites, collectively known as prostaglandin I metabolites or PGIM, (t1/2 = 30 minutes). Although 6-keto PGF1α is commonly measured in plasma and urine as an estimate of PGI2 synthesis, it should be noted that there may be more than one source of PGI2 in these samples. For instance, urinary concentrations of 6-keto PGF1α are confounded by the fact that only a portion originates from plasma and the remainder is produced by the kidney. These problems are circumvented by measuring downstream PGIM rather than 6-keto PGF1α as an indicator of systemic PGI2 production. Cayman's PGIM ELISA Kit is a competitive assay that can be used for quantification of PGIM in urine and other sample matrices. The assay has a range from 39-5,000 pg/ml and a sensitivity (80% B/B0) of approximately 120 pg/ml.