We report a new device and algorithm that allows simultaneous monitoring of the hematocrit and plasma volume fraction of blood within the intravascular space of an optically probed volume of skin. Skin is probed with a near infrared (NIR) laser and simultaneously collecting the Rayleigh and Mie scattered light as one raw signal and the undifferentiated Raman and fluorescence emission as the second raw signal. These signals are combined using six parameters that can be obtained by either direct calculation or empirical calibration to permit monitoring of the blood in human skin (e.g. fingertips). We tested a device based on the algorithm that might be useful in allowing the early detection of blood loss for people who have no external injury but may be hemorrhaging internally. IRB allowed experiments monitoring blood in human fingertip skin in vivo during routine hemodialysis demonstrated good agreement between the experimental device and the CRIT-LINE®, an FDA approved device that is built into the dialysis machine and applies the Twersky algorithm to blood in the dialysis machine (i.e. in vitro). Based on observation of 9 different test subjects, as dialysis removes fluid from the intravascular space causing an increase in hematocrit and a decrease in plasma volume, the CRIT-LINE response is closely emulated (typical per session linear correlation r2=0.78, N=87, p<0.0001) with the new device. Calibration across subjects, the measurement of absolute hematocrit, and potential confounding factors will also be discussed.