Objectives Therapeutic drug monitoring is performed routinely in patients on anti-epileptic drugs (AEDs) for optimisation and individualisation of therapy. The dried blood spot (DBS) sampling technique is a suitable, more patient-friendly alternative for conventional venous sampling methods. However, before DBS can be used in routine care, data are needed to establish the correlation between standard plasma concentrations obtained from venous puncture and concentrations measured through DBS obtained by finger prick. This study aims to investigate the correlation between carbamazepine, lamotrigine and levetiracetam drug concentrations in venous blood and DBS samples in the same patients at the same time.

Methods Clinical validation was conducted by direct comparison of paired DBS and venous plasma samples. Method agreement was evaluated using Passing–Bablok regression analysis and Bland–Altman plots to provide insight into the relationship between the two analytically validated methods. For Bland–Altman analysis the acceptance limit required by both FDA and EMA guidelines is at least two-thirds (67%) of the paired samples within 80–120% of the mean of both methods.

Results Paired samples from 79 patients were studied. For all three AEDs, plasma and DBS concentrations correlated highly (r=0.90 for carbamazepine, r=0.93 for lamotrigine and r=0.93 for levetiracetam), indicating a linear relationship. For carbamazepine and lamotrigine, no proportional or constant bias was revealed. For levetiracetam, concentrations were higher in plasma samples than in DBS (slope 1.21), implying a conversion factor is needed. The acceptance limit was met for carbamazepine and levetiracetam with a value of 72% and 81%, respectively. For lamotrigine, this acceptance limit was not met with a value of 60%.

Conclusions The method was successfully validated and will be used for therapeutic drug monitoring in patients using carbamazepine, lamotrigine and/or levetiracetam.