A markov model approach assessing the cost of illness of generalized anxiety disorder in Canada

Abstract

Introduction: Generalized anxiety disorder (GAD) is a chronic disease with waxing and waning of symptoms. This is the first comprehensive economic model developed to reflect the nature and course of GAD.

Methods: An incidence-based probabilistic Markov model was developed reflecting nine GAD health states (HS): clinical assessments (three HS), maintenance therapies (four HS), discontinuation (one HS), and death (one HS). A probability curve of the GAD onset (ages 18-80) determined entry into the model and assumed patients retained the diagnoses until death. Canadian Psychiatric Association (CPA) guidelines determined pharmacotherapy, with revisions/validation by an expert panel. Direct costs (clinician, pharmacotherapy, hospitalization) were retrieved from government publications. Remission was based on pooled-analysis of CPA-cited evidence. Remaining clinical rates, absenteeism, and hospitalization were retrieved from the literature. Direct costs were attributed throughout the model except for the discontinuation and death HS. Indirect costs (wage rate) were retrieved from government publications and the literature (absenteeism), and were attributed to patients with GAD ≤65 years of age. Results were discounted at 5% and results expressed in 2008 Canadian dollars.

Results: The mean lifetime cost of illness (COI) was estimated to be $31,213 (SD $9,100) per patient. The cost of absenteeism accounted for 96% of the mean COI. The mean age of onset was 31 years and approximately 19% did not respond to pharmacotherapy. Over 85% of patients discontinued treatment by the fourth cycle (2nd year of therapy). Over the course of the model, a mean of 53% of patients relapsed, with an average rate of 0.79 relapses per patient. On average and over a lifetime, the disorder went unmanaged over a period of 14 (SD 9) years. The model was most sensitive to absenteeism.

Conclusion: GAD is a costly disease with a lifetime COI <$32k/patient, with absenteeism exerting a significant impact.

Keywords: Absenteeism; Cost of illness; Decision model; Economics; Generalized anxiety; Markov model; Pharmacotherapy.

Fig. 1

Bubble diagram showing the health states and transition pathways for the Markov Model

Fig. 2

Decision tree within the Family Physician Assessment Health State (a), decision tree (continued from a) within the Family Physician Assessment Health State (b)

Fig. 3

Distribution of COI values from Monte Carlo simulation. COI cost of illness

Fig. 4

Health-state cumulative transition probabilities graph. Benzo benzodiazepine