A computer-implemented method for determining a level of hypertension of a person, said method comprising: acquiring image data defining a retinal image of the retinal vasculature of a retina of the person that has been captured by a retinal imaging system; processing the image data based on a classification algorithm, which has been trained on image data defining retinal images of the retinal vasculature of a plurality of subjects, to identify a plurality of vessel segments present in the retinal image corresponding to a plurality of portions of the retinal vasculature of the retina of the person; determining respective widths of vessel segments of at least a subset of the plurality of vessel segments present in the retinal image; calculating a value of a vascular summary metric using the determined widths of the vessel segments present in the retinal image, by: categorising each of the plurality of the vessel segments as an artery or non-artery; and calculating, as the value of the vascular summary metric and without making any estimate or assumption about a central retinal vasculature of the retina, a ratio of an average arterial width to an average venal width of the vessel segments present in the retinal image, wherein the average venule width is calculated using the widths of the vessel segments classified as non-artery; and determining the level of hypertension using the calculated value of the vascular summary metric and a predetermined relationship between the vascular summary metric and systolic blood pressure, wherein the vascular summary metric decreases with increasing systolic blood pressure. acquiring image data defining a retinal image of the retinal vasculature of a retina of the person that has been captured by a retinal imaging system; processing the image data based on a classification algorithm, which has been trained on image data defining retinal images of the retinal vasculature of a plurality of subjects, to identify a plurality of vessel segments present in the retinal image corresponding to a plurality of portions of the retinal vasculature of the retina of the person; determining respective widths of vessel segments of at least a subset of the plurality of vessel segments present in the retinal image; calculating a value of a vascular summary metric using the determined widths of the vessel segments present in the retinal image, by: categorising each of the plurality of the vessel segments as an artery or non-artery; and calculating, as the value of the vascular summary metric and without making any estimate or assumption about a central retinal vasculature of the retina, a ratio of an average arterial width to an average venal width of the vessel segments present in the retinal image, wherein the average venule width is calculated using the widths of the vessel segments classified as non-artery; and categorising each of the plurality of the vessel segments as an artery or non-artery; and calculating, as the value of the vascular summary metric and without making any estimate or assumption about a central retinal vasculature of the retina, a ratio of an average arterial width to an average venal width of the vessel segments present in the retinal image, wherein the average venule width is calculated using the widths of the vessel segments classified as non-artery; and determining the level of hypertension using the calculated value of the vascular summary metric and a predetermined relationship between the vascular summary metric and systolic blood pressure, wherein the vascular summary metric decreases with increasing systolic blood pressure.