Method of determining the presence and/or concentration of substances of interest in fluids

Magnetic particles coated with specific antibodies for substances of interest are introduced into a fluid sample. Over time and with agitation, the magnetic particles become attached to the substance of interest within the fluid. The fluid is then introduced to a magnetic field gradient provided between two magnets (1, 5). the resultant change to the magnetic filed between the magnets (1, 5) is determine by a Hall Effect device (3) or a plurality of Hall Effect devices (3). This enables the presence and/or concentration of the substance of interest in the sample to be determined.