A portable device for determining the oxygenation level in a human subject for the rapid diagnosis of poisoning by enzymatic toxic substances, such as cyanide, by means of an exam of an ocular fundus thereof by means of an indirect ophthalmoscopy technique associated with a camera, said device comprising: - a non-transparent, preferably cylindrical casing housing the following items: - a first light source (18', 18") to emit light at a wavelength of 805 nm and illuminate said ocular fundus, - a second light source (17', 17"), separated and distinct from said first light source, to emit light at a wavelength of 660 nm and illuminate said ocular fundus - a camera arranged to acquire images of the ocular fundus when illuminated by one of said first and second light sources, - elaborating means (9, 19) configured - to control said first and second light sources and said camera to acquire respectively first and a second images and - to apply an algorithm to the image acquired with red light at 660nm, based on the analysis of the intensity profile of circular areas around the optical disc, to identify the positions of retinal vessels on the image and to identify a list of sections of vessels and to classify them into sections of veins or arteries, and - to calculate for each of said first and second images and for each vessel section previously identified the respective optical densities at said two different wavelengths OD 805 and OD 660 as a logarithm of the ratio between the intensity I 0 of the pixels detected near the section of said veins or arteries and the intensity I of the pixels detected on the section itself and to calculate an ODR ratio of the optical densities at the two wavelengths as follows ODR = OD 660 / OD 805 to determine a level of oxygenation in the identified retinal veins. - a non-transparent, preferably cylindrical casing housing the following items: - a first light source (18', 18") to emit light at a wavelength of 805 nm and illuminate said ocular fundus, - a second light source (17', 17"), separated and distinct from said first light source, to emit light at a wavelength of 660 nm and illuminate said ocular fundus - a camera arranged to acquire images of the ocular fundus when illuminated by one of said first and second light sources, - elaborating means (9, 19) configured - to control said first and second light sources and said camera to acquire respectively first and a second images and - to apply an algorithm to the image acquired with red light at 660nm, based on the analysis of the intensity profile of circular areas around the optical disc, to identify the positions of retinal vessels on the image and to identify a list of sections of vessels and to classify them into sections of veins or arteries, and - to calculate for each of said first and second images and for each vessel section previously identified the respective optical densities at said two different wavelengths OD 805 and OD 660 as a logarithm of the ratio between the intensity I 0 of the pixels detected near the section of said veins or arteries and the intensity I of the pixels detected on the section itself and to calculate an ODR ratio of the optical densities at the two wavelengths as follows ODR = OD 660 / OD 805 to determine a level of oxygenation in the identified retinal veins.