A diagnostic patch apparatus has a sampling module that includes sampling means for sampling fluid from a patient's skin when the sampling module is placed against the patient's skin, and a sample chamber coupled in fluid communication with the sampling means. The apparatus also has an analysis module that includes a fluid conduit coupled in fluid communication with the sample chamber of the sampling module and a plurality of sensors coupled in fluid communication with the fluid conduit. The apparatus also may have a reader module that includes at least one optical sensor coupled in optical communication with the analysis module, a microcontroller coupled in electrical communication with the at least one sensor of the analysis module, and a wireless communication package coupled in electrical communication with the microcontroller.