How appropriate is therapeutic drug monitoring for lithium? Data from the Belgian external quality assessment scheme

Lithium remains a mainstay in the management of mood disorders. As with many psychotropic drugs, lithium treatment requires continuous observation for adverse effects and strict monitoring of serum concentrations. The present study aimed to assess the appropriateness of lithium assays used by Belgian laboratories, and to evaluate acceptability of their clinical interpretations.

Nine in-house serum samples spiked with predetermined concentrations of lithium were distributed to 114 participants in the Belgian external quality assessment scheme. Laboratories were requested to report the assay technique, lithium measurements and interpretations with regard to measured concentrations. Inter/intramethod imprecision and bias were reported and acceptability of clinical interpretations was assessed. The intramethod variability was evaluated by selecting methods used by 6 laboratories or more. Flame photometry (IL 943) was considered as the reference method.

Laboratories returned assay results using colorimetry (69.3%), ion selective electrode (15.8%), flame photometry (8.8%), atomic absorption spectroscopy (5.2%) or mass spectrometry (0.9%). Lithium concentrations were systematically higher when measured with the Vitros assay (median bias: 4.0%), and were associated with consecutive biased interpretations. In contrast, the Thermo Scientific Infinity assay showed a significant negative bias (median bias: 9.4%). 36.0% of laboratories reported numerical values below their manufacturer cut-off for the blank sample; 16.6% of these laboratories detected residual lithium concentrations.

The present study revealed assay-related differences in lithium measurements and their interpretations. Overall, there appeared to be a need to continue EQA of therapeutic drug monitoring for lithium in Belgium.