A substance determining apparatus determines a substance within a fluid where particles, which have attached the substance, are bound to a binding surface. A sensing unit is configured to generate a sensing signal being indicative of at least one of i) a distance between the particles bound on the binding surface and the binding surface, and ii) an in-plane position of the particles bound on the binding surface. A binding discrimination unit is configured to discriminate between different kinds of binding of the particles bound on the binding surface depending on the generated sensing signal. The binding discrimination unit may be a unit for determining the part of the sensing signal being caused by specifically bound particles and for determining the substance based on this determined part of the sensing signal.