Evaluating Appropriateness of Digoxin, Carbamazepine, Valproic Acid, and Phenytoin Usage by Therapeutic Drug Monitoring

BACKGROUND: Therapeutic drug monitoring (TDM) is a useful tool for the optimization of drug therapy. The aim of this retrospective study was to evaluate the appropriateness of carbamazepine, phenytoin, valproic acid, and digoxin therapy by using TDM data. METHODS: We evaluated the appropriateness of drug usage in 325 patients who received carbamazepine, valproic acid, phenytoin, or digoxin in a large teaching hospital during the period from March 2010 to January 2011. The serum drug levels were measured by cloned enzyme donor immunoassay (CEDIA). RESULTS: A total of 325 TDM tests were performed in this period. In our TDM unit, valproic acid was the most evaluated test among the samples obtained. In 100 valproic acid-treated patients, serum levels in 58 patients (58%) were within the therapeutic range. While 11 patients (11%) had serum levels in the above-therapeutic range, 31 patients (31%) had sub-therapeutic levels of valproic acid: The results of TDM were mostly found in the therapeutic range for carbamazepine. A total of 91 request forms were collected. The overall data show that 64 patients (70.3%) had serum carbamazepine levels within the therapeutic range. In the phenytoin assays, the mean plasma concentrations generally did not reach the therapeutic range. Among the total of 49 blood samples, the highest number of sub-therapeutic levels (65.3%) were detected for phenytoin. Similarly, inappropriate levels of digoxin were established in about half of all cases. CONCLUSIONS: This study gives information about the inappropriate usage of digoxin and phenytoin in respect to the results of our TDM practice. Our results suggest that there is a need for interventions to improve the appropriate use of digoxin and phenytoin in patients treated with these drugs.