Devices, systems, and methods for measuring tissue oxygen saturation (StO2) within the body are disclosed. A patient interface for use with a tissue measurement instrument can include a spring clip having a first arm and a second arm, a spring configured to bias the first and second arms together, and a means for optically connecting the patient interface to the tissue measurement instrument. The patient interface can be used in conjunction with an optical sourcing and receiving unit of a monitor for measuring tissue oxygen saturation at a measurement site on the patient. A testing interface on an external portion of the instrument housing and a testing module can be used to perform tests on the instrument.