PGE2 is metabolized rapidly to 13,14-dihydro-15-keto PGE2, which is present in the plasma of humans and other mammals. 13,14-dihydro-15-keto PGA2 results from the non-enzymatic dehydration of 13,14-dihydro-15-keto PGE2, a process which is accelerated by the presence of albumin. Further decomposition of 13,14-dihydro-15-keto PGA2 by the intentional addition of base produces bicyclo PGE2, a stable marker of PGE2 biosynthesis.