An apparatus is provided for testing fluid samples includes a sensor, which can be light source, directed to a flow cell and a photo sensor for detecting a light beam reflected from the flow cell. The photo sensor monitors the fluid in the flow cell by sensing the reflected light beam from the flow cell, thereby monitoring the test process. The apparatus may have additional light source so that the photo sensor may monitor the test process by detecting the absorption light beam or fluorescent light beam from the flow cell at different phases of the test process.