In addition to a direct determination of PGD2 and/or its metabolite 9α,11β-PGF2α we have recently introduced an in vitro test in which substance-P-triggered release of PGD2 from peripheral blood leukocytes unambiguously distinguishes MCAD patients from healthy individuals [36]. This evidence concerns the gene TAC1 and medium chain acyl-CoA dehydrogenase deficiency.