Disparities in the medical expenditures of patients with cancer and concomitant mental disorder: analyzing the effects of diagnosis sequence order

BACKGROUND: Cancer is the leading cause of death in Taiwan. Medical expenditures related to cancer accounted for 44.8% of all major illness insurance claims in Taiwan. Prior research has indicated that the dual presence of cancer and mental disorder in patients led to increased medical burden. Furthermore, patients with cancer and concomitant mental disorder could incur as much as 50% more annual costs than those without. Although previous studies have investigated the utilization of patients with both diseases, the effects of morbidity sequence order on patient costs are, however, uncertain. This study explored medical expenditures linked with the comorbidity of cancer and mental disorder, with a focus on the impact of diagnosis sequence order. METHODS: This population-based retrospective matched cohort study retrieved patients with cancer and mental disorder (aged ≥ 20 years) from the Ministry of Health and Welfare Data Science Center 2005–2015 database. 321,045 patients were divided based on having one or both diseases, as well as on the sequence of mental disorder and cancer diagnosis. Study subjects were paired with comparison counterparts free of both diseases using Propensity Score Matching at a 1:1 ratio. Annual Cost per Patient Linear Model (with a log-link function and gamma distribution) was used to assess the average annual cost, covarying for socio-demographic and clinical factors. Binomial Logistic Regression was used to evaluate factors associated with the risk of high-utilization. RESULTS: The “Cancer only” group had higher adjusted mean annual costs (NT$126,198), more than 5-times that of the reference group (e^β: 5.45, p < 0.001). However, after exclusion of patients with non-cancer and inclusion of diagnosis sequence order for patients with cancer and concomitant mental disorder, the post-cancer mental disorder group had the highest expenditures at over 13% higher than those diagnosed with only cancer on per capita basis (e^β: 1.13, p < 0.001), whereas patients with cancer and any pre-existing mental disorder incurred lower expenditures than those with only cancer. The diagnosis of post-cancer mental disorder was significantly associated with high-utilization (OR = 1.24; 95% CI: 1.047–1.469). Other covariates associated with high-utilizer status included female sex, middle to old age, and late stage cancer. CONCLUSION: Presence of mental disorder prior to cancer had a diminishing effect on medical utilization in patients, possibly indicating low medical compliance or adherence in patients with mental disorder on initial treatments after cancer diagnosis. Patients with post-cancer mental disorder had the highest average annual cost. Similar results were found in the odds of reaching high-utilizer status. The follow-up of cancer treatment for patients with pre-existing mental disorders warrants more emphasis in an attempt to effectively allocate medical resources. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12913-023-09056-9.