Clinical tests which are displayed are based on a system-theoretic approach for an early diagnosis of the immunological and virological failure of HIV patients. Mathematical characterizations of therapeutical failures are presented in this paper. Mathematical modeling is used for individual patients to help for an early diagnosis of the evolution of the infection. The feasibility of the method is depicted on some patients who start highly active antiretroviral therapy (HAART). The identifiability of the continuous-time models which are used is proved, and it is shown to be invariant under discretization.