devices and methods for measuring body fluid - related metric using spectrophotometry that may be used to facilitate diagnosis and therapeutic interventions aimed at restoring body fluid balance . in one embodiment , the present invention provides a device for measuring a body - tissue water content metric as a fraction of the fat - free tissue content of a patient using optical spectrophotometry . the device includes a probe housing configured to be placed near a tissue location which is being monitored ; light emission optics connected to the housing and configured to direct radiation at the tissue location ; light detection optics connected to the housing and configured to receive radiation from the tissue location ; and a processing device configured to process radiation from the light emission optics and the light detection optics to compute the metric where the metric includes a ratio of the water content of a portion of patient &# 39 ; s tissue in relation to the lean or fat - free content of a portion of patient &# 39 ; s tissue .