Electrochemical Monitoring of Tacrolimus in Whole Blood: A Step Toward Decentralized Therapeutic Drug Monitoring

Tacrolimus (TAC) is a cornerstone immunosuppressive drug used in organ transplantation that requires strict therapeutic drug monitoring due to its narrow therapeutic window and high interindividual variability. Current monitoring relies mainly on immunoassays and liquid chromatography coupled to tandem mass spectrometry (LC–MS/MS). Although LC–MS/MS provides high analytical specificity, its implementation is largely restricted to centralized laboratories due to the need for specialized instrumentation and trained personnel, whereas immunoassays may suffer from metabolite cross-reactivity and method-dependent bias. Consequently, there is a clinical need for alternative analytical strategies that combine sensitivity within the therapeutic concentration range with simpler instrumentation and workflows suitable for decentralized testing. Herein, we report an electrochemical sensor based on a carbon black–modified screen-printed electrode for tacrolimus determination, with application to TAC-spiked whole blood. The sensing strategy exploits the adsorptive stripping voltammetry technique, enabling analyte preconcentration and enhanced sensitivity without biological recognition elements. Under optimized conditions, the sensor achieved a limit of detection of 0.5 ng mL−1 in buffer, with a quantitative linear range of 1.8–100 ng mL−1. Following a simple local pH-adjustment step, the platform was evaluated in TAC-spiked whole blood, achieving a limit of detection of 0.7 ng mL−1 and a quantitative linear range of 2.5–37.5 ng mL−1. Overall, the proposed platform combines sub-ng mL−1 sensitivity with a simple, disposable, and recognition-element-free electrode configuration, providing a promising basis for further development toward decentralized tacrolimus therapeutic drug monitoring.