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Background: Estimation of serum concentrations of antiseizure medications (ASMs) based on dried capillary blood is an alternative method for therapeutic drug monitoring of epilepsy. The aim of this study was to validate the conversion factors for lacosamide (LCM), lamotrigine (LTG), and levetiracetam (LEV), which were determined in an independent patient sample in a previous study, and identify the most accurate conversion method (simple ratio and regression)., Methods: Venous and capillary blood samples were collected from adult inpatients with epilepsy treated with LCM (n = 25), LTG (n = 27), and/or LEV (n = 29) before the morning dose (T1) and approximately 2 hours after (T2). Capillary blood was collected using volumetric absorptive microsampling, and the ASM concentrations were measured using a validated liquid chromatography-mass spectrometry method for dried blood samples. Serum concentrations were estimated using conversion factors and compared with those measured using routine laboratory methods., Results: For all 3 ASMs, the simple ratio approach performed better than the regression approach. Intraclass correlation coefficients revealed a high agreement between the estimated and measured serum concentrations (LCM T1: 0.93, T2: 0.90; LTG T1: 0.91, T2: 0.91; and LEV T1: 0.97, T2: 0.94). The criteria of the European Medicines Agency for cross-validation were fulfilled for LCM (T1: 72%; T2: 75%) and LEV (T1: 86%; T2: 75%), whereas for LTG, this was only true for capillary blood concentrations ≤11 µ g/mL [42.9 µ mol/L; T1: 72% (vs. 63% for total range), T2: 67% (vs. 62%)]., Conclusions: Estimating serum concentrations using capillary blood concentrations is feasible and accurate for LCM and LEV over a wide concentration range, as found in clinical practice. The applicability of this mehod for LTG is limited by its greater variability at higher concentrations; however, acceptable results were achieved for the large proportion of patients with low and medium LTG concentrations., Competing Interests: The authors declare no conflict of interest., (Copyright © 2023 Wolters Kluwer Health, Inc. All rights reserved.)

Background: Drug concentrations of antiepileptic drugs (AEDs) are routinely determined from blood serum or plasma at trough levels (before intake of morning dose). In capillary blood collection, blood is taken from the fingertip with the aid of a disposable tool and dried on absorbent material. The volumetric absorptive microsampling technique offers several advantages over the use of filter paper cards. The aim of this study was to determine conversion factors for the estimation of AED serum concentrations from capillary blood concentrations., Methods: Venous and capillary blood samples were collected from adult inpatients with epilepsy who were treated with lacosamide (LCM, n = 30), lamotrigine (LTG, n = 40), and/or levetiracetam (LEV, n = 36). A validated liquid chromatography-mass spectrometry (LC-MS) method for dried blood samples for these AEDs was compared with routine serum laboratory methods. Method agreement was evaluated using different regression techniques, and the conversion factors were calculated., Results: Regression analyses revealed a linear relationship between serum and capillary blood concentrations for all 3 AEDs (r ≥ 0.95). For LTG, the regression intercept was significantly different from 0, indicating that the relationship was linear, but not necessarily proportional. Although LEV and LCM concentrations tended to be lower in capillary blood than in serum (mean ratio of serum concentration to capillary blood concentration: 1.14 and 1.22, respectively), LTG concentrations were higher in capillary blood (mean ratio = 0.85)., Conclusions: The estimation of serum concentrations from measured capillary blood concentrations is feasible for LCM, LTG, and LEV. A simple ratio approach using the mean ratio and Passing-Bablok regression showed the best results for all 3 AEDs. The volumetric absorptive microsampling technique facilitates the quantitative sample collection of capillary blood and overcomes the drawbacks associated with the classical dried blood spot technique., Competing Interests: C. G. Bien obtained honoraria for speaking engagements from UCB (Monheim, Germany) and Desitin (Hamburg, Germany). He receives research support from the Deutsche Forschungsgemeinschaft (German Research Council, Bonn, Germany) and Gerd-Altenhof-Stiftung (Deutsches Stiftungs-Zentrum, Essen, Germany). Christian Brandt has received personal compensation from Arvelle Therapeutics (Zug, Switzerland), Desitin (Hamburg, Germany), Eisai (Woodcliff Lake, NJ), GW Pharmaceuticals (Histon, United Kingdom), Idorsia (Basel, Switzerland), and UCB Pharma (Monheim, Germany) for consulting services or speaking activities. The remaining authors declare no conflict of interest., (Copyright © 2021 Wolters Kluwer Health, Inc. All rights reserved.)