We describe an opto-electronic device capable of measuring the hemoglobin concentration (Hgb) non-invasively and continuously, hence the percentage changes in blood volume (BV) during dialysis treatment by means of the optical absorption of monochromatic light by the blood in the arterial line. This method has been validated during several in vitro and in vivo tests, during which the system has shown a low sensitivity to all the common intra-dialytic interference factors, such as oxygen saturation (max. err. = 1.6%), blood flow (max. err. = 1.8%), osmotic pressure (max. err. = 0.7%) and hydraulic pressure (max. err. = 0.6%), a high precision (std. err. < 0.1 g/dl) and a good accordance (Hgb mean err. = 0.1 g/dl; std. err. = 0.38 g/dl; BV mean err. = 0.1%; std. err. = 1.6%) with the corresponding values derived from standard laboratory tests.