Surrogate-based LC-MS/MS quantification of endogenous creatinine in plasma, plasma-equivalent dried spots (qDPS), and...

Creatinine quantification in biological matrices remains analytically challenging due to its endogenous presence, which precludes the use of conventional external calibration methods. Concurrently, microsampling techniques such as plasma-equivalent dried plasma spots (qDPS) and volumetric absorptive microsampling (VAMS) are gaining increasing popularity for decentralized, low-volume, patient-centred sampling. Nevertheless, robust cross-matrix strategies for endogenous analytes remain limited. This study aimed to develop and validate an ICH M10-compliant LC-MS/MS method based on a surrogate analyte approach for creatinine determination in plasma, qDPS, and VAMS, and to assess analytical bridging between conventional plasma and microsamples. Calibration exhibited linearity over the range of 0.025-100 mg/L, with R² values of 0.9982, 0.9964, and 0.9971 for plasma, qDPS, and VAMS, respectively. Intra- and inter-day precision were ≤15% (≤20% at the LLOQ), and accuracy remained within ±15% at all quality control levels. Carryover was maintained within acceptable limits (<20% of the LLOQ for creatinine and <5% for the internal standard), while matrix-related variability was effectively controlled through isotopically labelled internal standardisation. Parallelism verified comparable analytical behaviour across matrices and hematocrit levels of 0.21-0.68 L/L. Creatinine remained stable in all matrices for 5 h at 60 °C, whereas microsamples preserved stability for up to 3 months under ambient, refrigerated, and frozen conditions. In a cohort of 61 renal transplant recipients, mean plasma creatinine concentrations were 1.57 mg/dL, 1.54 mg/dL in qDPS, and 1.26 mg/dL in VAMS. qDPS yielded plasma-equivalent concentrations without any need for recalculation, whereas VAMS necessitated regression-based correction (PL_CRE = 1.197 × VAMS_CRE+0.053). Collectively, these findings establish a validated surrogate-based LC-MS/MS platform for the precise measurement of creatinine across plasma and volumetric microsampling matrices, thereby providing a reliable analytical foundation for decentralized monitoring of renal function during transplant follow-up.