Magnetic nanoparticles are detected across a thin membrane that separates the nanoparticles from a magnetic sensor. The technique can be used in a medical context, in which an analyte of interest (present in a test fluid, such as blood) is attached to the membrane. Other compounds are in turn bound to the analyte, with one of these compounds including a magnetic nanoparticle that is then detected by the sensor. In this way, the analyte is detected by detecting the magnetic nanoparticle. By counting the number of magnetic nanoparticles, the concentration of the analyte in the test fluid can be determined.