To assist diagnosis of a post-operative condition related to an implanted medical device, such as an infection, inflammation, mechanical wear, and hemorrhaging, a monitoring device optically measures a concentration of a selected type of cells in a fluid surrounding the implanted medical device. The monitoring device comprises a light source and light sensor which are selected and calibrated, and placed in a relative spatial orientation with respect to each other, to detect light scattered by the selected type of cell in the fluid.
