The invention relates to a substance determining apparatus for determining a substance within a fluid. Particles attach the substance and bind to a binding surface (30), wherein a sensing signal is generated depending on the bound particles. Binding events indicating a binding of a particle on the binding surface (30) are determined from the generated sensing signal, and the substance within the fluid is determined based on the determined binding events. During a procedure of determining a substance within a fluid, particles may bind to the binding surface and may leave the binding surface. Therefore, during this procedure a number of binding events can be determined being much larger than the number of bound particles. The determination of the substance within the fluid can therefore be based on a very large amount of data, thereby increasing the accuracy of determining the substance within the fluid.