Robust UPLC–MS/MS Method With Acetonitrile for Precise Intracellular Quantification of Tacrolimus in PBMCs: A Step Toward Clinical Integration

= 0.999). Intra- and interday accuracy ranged from 98.1% to 109.8%, with precision between 2.08% and 8.70% across validation runs. Extraction recovery was high (93.0%-97.2%), with minimal matrix effects (100.9% at low QC and 111.6% at high QC). This validated LC-MS/MS method provides a rapid, reliable, and sensitive approach for pharmacokinetic studies and clinical applications, facilitating intracellular tacrolimus monitoring in transplant patients.