Clinical Application of Microvolume LC–MS/MS for Therapeutic Drug Monitoring of Immunosuppressants in Solid-Organ Transplant Recipients

Background/Objectives: Therapeutic drug monitoring (TDM) is essential for optimizing immunosuppressive therapy in solid-organ transplant recipients by maintaining efficacy, while minimizing adverse effects. However, conventional TDM relies on venous sampling and separate assays for tacrolimus (TAC) in whole blood and mycophenolic acid (MPA) in plasma, thereby increasing patient burden and procedural complexity. To address these limitations, we investigated the clinical utility of a microvolume, liquid-phase microsampling device (MSW2™) in combination with liquid chromatography–tandem mass spectrometry (LC-MS/MS). Methods: We established and applied an LC-MS/MS method for simultaneous quantification of TAC, MPA, and mycophenolic acid β-D-glucuronide (MPAG) using only 2.8 µL of whole blood collected with MSW2™, which eliminates drying or extraction steps. Hematocrit-based correction was applied to estimate plasma MPA concentrations from whole-blood measurements. The method was evaluated in 60 renal transplant recipients with paired venous samples for comparison. Analytical performance was assessed using regression, Bland–Altman analyses, predictive metrics, and stability testing under different storage conditions. Results: Microsampled and venous concentrations were strongly correlated (R2 > 0.95). Estimated plasma MPA concentrations derived from whole blood closely approximated plasma concentrations (bias < 5%). Reducing the sample volume from 5.6 µL to 2.8 µL improved precision and increased the success rate of blood collection from 72.9% to 94.0%. All analytes remained stable for up to 72 h at ≤25 °C. Conclusions: This approach enables accurate, simultaneous quantification of multiple immunosuppressants from trace blood volumes. By reducing sampling burden and simplifying logistics, it provides a clinically feasible and patient-centered strategy for precision TDM, supporting broader implementation of limited sampling strategies and expanding applicability to pediatric, home-based, and telemedicine settings.