Device and method for monitoring body fluid and electrolyte disorders

A device (400; 500; 600) for measuring body fluid-related metrics using optical spectrophotometry comprising: a probe housing configured to be placed in a location proximal to the tissue being monitored; optical light emission elements (618) connected to said housing and configured to direct radiation at said tissue location; detection optics of light (620) connected to said housing and configured to receive radiation from said location of tissues, and a processing device (624) configured to process optical radiation received from said optical emitting elements light (618) and said detection optics of light (620) to calculate the metrics mentioned related body fluids, said processing device (624) being adapted to receive and compare at least two sets of optical measurements to least two different wavelengths, characterized in that the absorption of light in said at least two different wavelengths is primarily due to water which is in the vascular blood and in the extravascular tissue, and where a comparison of said at least two measurements provides a measure of the difference between the fractions of water in the blood and surrounding tissue location.