A method and apparatus to monitor clinical states, for example hypoglycaemia, hyperglycaemia or an ischemic state, and to detect if a user if affected by such a state. In one embodiment, the method and apparatus use a correlation between one or more non-invasive physical measurements of the body of the subject and a reference value of blood-glucose of the subject to compute an estimation parameter value, and comparing that estimation parameter value with a critical parameter value indicative of the presence of a clinical state, for example hypoglycaemia, hyperglycaemia. In another embodiment, the algorithm estimates chromophore concentrations and the scattering coefficient in order to compute a level of oxygen saturation and detect the presence of an ischemic state.