Source: extracted from pages 30 to 33, Chapter 2 of "Decision Modelling for Health Economic Evaluation" by Briggs, Claxton & Sculpher

A case study in HIV

The details of the Markov model can be illustrated using a case study. This is a cost-effectiveness analysis of zidovudine monotherapy compared with zidovudine plus lamivudine (combination) therapy in patients with HIV infection (Chancellor et al. 1997). This example has been used for didactic purposes before (Drummond et al. 2005), but is further developed here, and in Chapter 4 for purposes of probabilistic analysis.

The structure of the Markov model is shown in Fig. 2.3. This model characterizes a patient's prognosis in terms of four states. Two of these are based on CD4 count: 200-500 cells/mm3 (the least severe disease state - State A) and less than 200 cells/mm3 (State B). The third state is AIDS (State C) and the final state is death (State D). The arrows on the Markov diagram indicate the transitions patients can make in the model. The key structural assumption in this early HIV model (now clinically doubtful, at least in developed countries) is that patients can only remain in the same state or progress; it is not feasible for them to move back to a less severe state. More recent models have allowed patients to move back from an AIDS state to non-AIDS states and, through therapy, to experience an increase in CD4 count. These models have also allowed for the fact that prognosis for these patients is now understood in terms of viral load as well as CD4 count (Sanders et al. 2005).