Quality Assurance when Using Therapeutic Drug Monitoring: A Lamotrigine Split Sample Method to Reflect Comparability

Abstract

Purpose: Therapeutic drug monitoring can optimise patient outcomes if the Anti-Epileptic Medication (AEM) level is received in a timely fashion. When choosing a laboratory (lab) to measure levels, the treating physician must incorporate quality assurances so as to be confident that the results are reliable and concordant with results accepted from the current lab. The study aims to generate a practical example of how one can improve the use of drug monitoring in patients with epilepsy. Methods: A split-sampling procedure was used to analyse the AEM levels reported by two different labs. The results were categorised in accordance with the physician’s defined therapeutic range: sub-therapeutic: <10mg/L; therapeutic: 10-16mg/L; and supra-therapeutic: >16mg/L, to determine if categorisation varied between the labs. Results were further evaluated to compensate for absolute and/or clinically significant differences. Results: Categories were concordant for 43/50 (86%) of results. Of the 7/50 (14%) category discordant results, five (10% of results) were not clinically significant. In only 4% (2/50) of patients was the discordance sufficient to have possibly generated a treatment modification depending upon the patient’s clinical picture. Overall, the absolute difference in the levels reported by the two labs was neither significant nor statistically different. Conclusion: Split-sampling studies are a practical way of ensuring physician confidence and demonstrating quality assurance when changing labs.