11β-PGF2α is the primary plasma metabolite of PGD2 in vivo, the levels of which can increase from 6 pg/ml in a normal healthy volunteer to 490 ng/ml in patients with systemic mastocytosis. The normal human urinary excretion of 11β-PGF2α is about 11 ng/mmol creatinine (~400 ng/24 hr), which is increased nearly 3-fold upon allergen-induced bronchoconstriction in asthmatics. Cayman’s 11β-PGF2α Assay is a competitive assay that provides accurate measurements of 11β-PGF2α within the range of 1.6-1,000 pg/ml.