Bicyclo PGE2 is a stable, base-catalyzed transformation product of 13,14-dihydro-15-keto PGE2. 13,14-dihydro-15-keto PGE2 itself is a metabolite of PGE2{182} found in human plasma at a median level of 20-25 pg/ml.{4383} Due to the inherent instability of 13,14-dihydro-15-keto PGE2, it is advisable to quantify it as bicyclo PGE2 to estimate PGE2 biosynthesis or metabolism in vivo.{2571,581}