A preparation for determining levodopa metabolic capacity, comprising as an active ingredient a levodopa in which at least one of the carbon or oxygen atoms is labeled with an isotope, wherein the preparation is capable of producing isotope-labeled CO 2 after administration to a mammalian subject. A method for determining levodopa metabolic capacity in a mammalian subject, comprising the steps of administering a preparation according to claim 1 to a mammalian subject, and measuring the excretion behavior of an isotope-labeled metabolite excreted from the body. A method for determining the efficacy of a dopamine decarboxylase inhibitor to treat a medical condition in a first mammalian subject, the method comprising (a) administering to the first subject the dopamine decarboxylase inhibitor and a levodopa in which at least one of the carbon or oxygen atoms is labeled with an isotope, wherein the levodopa is capable of producing isotope labeled CO 2 ； (b) determining the level of levodopa metabolic capacity by measuring isotope labeled CO 2 produced in the first subject； and (c) comparing the level of levodopa metabolic capacity of the first subject to level of a reference standard levodopa metabolic capacity, wherein a similarity in the level of levodopa metabolic capacity of the first subject compared to level of a reference standard levodopa metabolic capacity indicates that the dopamine decarboxylase inhibitor is effective to treat the medical condition in the first mammalian subject.
