Routine outpatient L-dopa monitoring in elderly patients with Parkinson's disease

In an attempt to improve the therapeutic drug management of patients with Parkinson's disease, plasma L-dopa concentrations were measured by high-performance liquid chromatography and related to the post-dose time with reference to an established therapeutic range of 0.3-1.6 mg/l. The response to treatment was also assessed. One hundred and three samples were obtained at morning clinics from 53 elderly patients (mean age 72.5 years) taking an L-dopa/decarboxylase inhibitor combination. L-Dopa concentrations ranged from 0.01 to 3.6 mg/l. Fifty-nine values were within, 39 values were below and five values were above the therapeutic range. Three values were at or below the lower limit of the assay and probably indicated poor compliance. L-Dopa concentration was significantly negatively correlated with post-dose time for the dosage groups of 50 mg (p = 0.04), 100 mg (p = 0.0013), 200-250 mg (p = 0.055) and for the combined data (p = 0.005). Post-dose times were from 35 to 400 min, with the majority greater than 90 min, and it is likely that most of these corresponded to the post-peak phase of L-dopa absorption. There was a tendency for a good response to treatment to occur with values within and above the therapeutic range and for dyskinesia to be more common above the therapeutic range. It was concluded that plasma L-dopa measurement at known post-dose time, 90-360 minutes after the morning dose, can identify non-compliance, patients at risk of dose related side-effects and give useful information about the suitability of the L-dopa dose.