Cost of Illness Study

A cost of illness (COI) study estimates the total economic burden that a specific disease or health condition imposes on society, the health care system, patients, or payers. COI studies quantify three main categories of costs: direct medical costs (hospitalizations, physician visits, medications, diagnostic tests, rehabilitation), direct nonmedical costs (transportation to appointments, home modifications, informal caregiving), and indirect costs (lost wages, reduced productivity, premature mortality). By documenting the economic magnitude of a disease, COI studies inform priority setting, resource allocation, and the justification for investing in prevention and treatment research.

COI studies provide the economic context that decision makers need to understand why investing in a particular disease area matters. A high economic burden shows where effective interventions could make a large difference, although a COI study does not compare interventions, so it cannot show whether any particular intervention is good value; that needs an economic evaluation. Funding agencies use COI data to justify research investments; health systems use them to plan budgets and allocate resources; and advocacy groups use them to raise awareness about underfunded conditions. COI estimates also serve as inputs to cost effectiveness and cost benefit analyses, providing the baseline costs against which the economic impact of new interventions is measured.

COI studies can take either a prevalence based or an incidence based approach. Prevalence based studies estimate the total costs attributable to a disease in a given year, summing the costs incurred by all people living with the condition during that period. Incidence based studies estimate the lifetime costs from the point of diagnosis for a cohort of newly diagnosed individuals. Data sources include administrative claims databases, hospital discharge records, national health surveys, disease registries, and published literature. Costs are calculated by multiplying resource use quantities (number of hospitalizations, physician visits, prescriptions) by unit prices, with adjustments for inflation using health care price indices. Indirect costs are typically estimated using the human capital approach (valuing lost productivity at market wages) or the friction cost method (limiting indirect cost estimates to the period needed to replace an absent worker).

Conduct a COI study when you need to establish the economic magnitude of a disease to support research funding applications, policy advocacy, health system planning, or the development of economic evaluation models. COI studies are particularly valuable for conditions that are underrecognized or underfunded, where demonstrating the economic burden can stimulate investment. They are also useful for comparing the relative burdens of different diseases within a health system or for tracking how the economic burden of a condition changes over time.

A common error is double counting costs by including the same expenditure in multiple categories. Another mistake is omitting important cost categories, particularly indirect costs and informal caregiving costs, which can constitute the majority of the economic burden for chronic conditions. Researchers sometimes use inappropriate comparison populations or fail to separate disease attributable costs from costs that patients would have incurred regardless of their condition. The choice of cost valuation method (charges versus costs, human capital versus friction costs) significantly affects the estimates, and researchers should justify their choices and report sensitivity analyses. Finally, extrapolating COI estimates from one country or health system to another without adjusting for differences in prevalence, treatment patterns, and prices produces misleading results.

A research team estimates the cost of diagnosed diabetes in the United States in 2017 using a prevalence based approach, combining national surveys such as the Medical Expenditure Panel Survey and the National Health Interview Survey with claims data. They estimate that 24.7 million people have diagnosed diabetes. Total estimated costs are $327 billion: $237 billion in direct medical costs (hospital inpatient care 29%, prescription medications other than diabetes medicines 30%, diabetes medicines and supplies 15%, physician office visits 13%, other 13%) and $90 billion in indirect costs (absenteeism, reduced productivity at work and at home, inability to work because of disability, and premature mortality). People with diagnosed diabetes average $16,752 a year in medical spending, about 2.3 times the $7,151 expected for people of the same age and sex without diabetes, so about $9,601 per person per year is attributed to diabetes. These estimates help justify continued federal investment in diabetes prevention and management programs.