An in-vivo blood composition analyzing apparatus and method generates a laser beam at the blood vessels of the patient's eye. The Raman scattered photons from the blood vessels are collected and used to generate a Raman spectrum indicative of the blood composition. In one embodiment, a laser beam is generated with two streams of photons, the photons of one beam being entangled with the photons of the other beam. One stream is then directed at the eye and only the other beam is analyzed to obtain said Raman spectrum. Alternatively, the scattered photons from the second beam are also analyzed and the results are correlated for greater efficiency and accuracy.