Molecular communication systems enable promising applications, such as the monitoring of the human organs at nanoscale. An open issue in this context is the design of a practical interface between the biological system inside the body and an RF-based backhaul, which would enable an uplink connection to a central processing unit. In this letter, we present an interface based on magnetic nanoparticles released by a nanomachine inside a blood vessel of a finger and sensed by an attached susceptometer. Our numerical evaluations show that the proposed method achieves a high performance in terms of detection accuracy and power consumption.