Toward noninvasive measurement of blood hematocrit using spectral domain low coherence interferometry and retinal tracking.

We demonstrate in vivo measurements in human retinal vessels of an experimental parameter, the slope of the low coherence interferometry (LCI) depth reflectivity profile, which strongly correlates with the real value of blood hematocrit. A novel instrument that combines two technologies, spectral domain low coherence interferometry (SDLCI) and retinal tracking, has been developed and used for these measurements. Retinal tracking allows a light beam to be stabilized on retinal vessels, while SDLCI is used for obtaining depth-reflectivity profiles within the investigated vessel. SDLCI backscatter extinction rates are obtained from the initial slope of the A-scan profile within the vessel lumen. The differences in the slopes of the depth reflectivity profiles for different subjects are interpreted as the difference in the scattering coefficient, which is correlated with the number density of red blood cells (RBC) in blood. With proper calibration, it is possible to determine hematocrit in retinal vessels. Ex vivo measurements at various RBC concentrations were performed to calibrate the instrument. Preliminary measurements on several healthy volunteers show estimated hematocrit values within the normal clinical range.