Simple lithium measurement in capillary dried plasma microsamples collected with the HealthID PSD device

Monitoring serum lithium concentrations is essential during bipolar disorder treatment. This study aimed to develop and validate a simple approach for estimating serum lithium concentrations using dried plasma spots (DPS) obtained from the HealthID PSD device after application of capillary blood.

Lithium was extracted using BSA solution and quantified with a colorimetric assay on an automated clinical analyzer. A chloride-based correction factor was applied to account for variable plasma volumes in DPS extracts. Analytical validation included linearity, extraction yield, imprecision, hematocrit and volume effects, chromatographic uniformity during plasma migration, and stability at different temperatures. A clinical comparison study was performed using paired serum samples and DPS from 40 patients undergoing lithium therapy. Serum-equivalent lithium concentrations were calculated by applying a multiplication factor derived from the serum-to-DPS ratio. Agreement was assessed by Passing-Bablok regression, Bland-Altman analysis, median percentage predictive error (MPPE), median absolute percentage error (MAPE), and total error (TE).

Linearity was demonstrated from 0.1-2.5 mmol/L. Extraction yields were 93.2-95.7 %, and chloride correction significantly improved imprecision. Hematocrit (30-50 %) and applied volume (160-240 µL) showed minimal influence on corrected concentrations. Lithium remained stable for 21 days. Serum-equivalent DPS lithium showed strong correlation with serum (r = 0.984), no proportional or systematic bias, MPPE 0.91 %, MAPE 6.7 %, and TE 11.9 %.

Lithium can be reliably quantified in DPS generated with the HealthID PSD device using a routine colorimetric assay. The method enables stable, minimally invasive capillary microsampling suitable for clinical monitoring of lithium therapy.