Measuring L-dopa in plasma and urine to monitor therapy of elderly patients with Parkinson disease treated with L-dopa and a dopa decarboxylase inhibitor.

We have established a method for measuring L-dopa in plasma and urine, including the metabolites dopamine and L-dopac, using separation by ion-pair reversed-phase HPLC and quantification with an electrochemical detector. The assay was applied to the therapeutic monitoring of elderly patients with established Parkinson disease being treated with L-dopa plus a dopa decarboxylase inhibitor. Plasma L-dopa was evaluated in relation to dosage and postdose sampling time in 71 outpatients with Parkinson disease. L-Dopa concentrations were greatest in the patients taking the highest dosages prescribed and decreased significantly with increasing time after postdose sampling. Comparison of plasma L-dopa concentrations with a published therapeutic range established by intravenous administration of L-dopa was helpful in assessing the suitability of each patient's drug dosage, assessing patients' compliance, and avoiding overdosage but was not useful in the overall clinical assessment of progression of disease or of the long-term therapeutic response. Urine measurements confirmed the plasma concentrations but showed no further advantage. The recommended time for sample collection is between 1.5 and 3 h after the first morning dose. Plasma is the preferred matrix but if blood sampling is difficult, particularly from elderly/infirm individuals, an untimed urine collection could be used.