Original articles
The cost diary: a method to measure direct and indirect costs in cost-effectiveness research

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Abstract

From a societal perspective long-term clinical trials or follow-up studies should preferably not only include an evaluation of the health effect for the patient, but also an economic evaluation. In order to yield comprehensive medical and nonmedical resource use data, we at least partly depend on respondents' recall for collecting these costing data. A patient cost diary was developed in order to estimate total resource use, expenses, and lost production due to illness and treatment. We applied the cost diary in two randomized clinical trials evaluating the cost-effectiveness of behavioral rehabilitation in 205 fibromyalgia and chronic low back pain patients. The use of the diary was evaluated, studying the feasibility, the influence of the period of data collection on the results, and some aspects of validity. Eighty-five percent of the patients completed at least one diary and in total 68% of the diaries were returned. Although the results for the three alternative periods of data collection (keeping the diary 1 week every month, 2 weeks every 2 months, or a full year) were not significantly different, they were only moderately correlated. Finally, self-reported specialist care contacts were generally in agreement with data from an insurance company. However, for physiotherapy contacts there were differences between the self-reported and insurance data. This study shows how the cost diary might be used successfully in cost-effectiveness studies.

Introduction

It is widely recommended that cost-effectiveness studies be conducted from the societal perspective. From this broad perspective, all costs and savings are considered. This means that studies designed to investigate cost-effectiveness should base their conclusions upon all relevant medical and nonmedical costing data that they are able to collect. However, apart from the productivity costs, the cost for nonmedical goods and services associated with an intervention are rarely included, because collecting this information is complex and demands careful scheduling in appraisals covering long-term care 1, 2. Especially in evaluation studies of treatment for most chronic disorders, data collection should not be limited to the medical costs. Chronic diseases may not only generate a large and growing need of medical care, but they may also have a profound socioeconomic impact on the patients, their families, and their social and working relations.

In the context of cost-effectiveness analyses linked to a clinical trial, there are various sources to obtain data, such as patient records, data bases from insurance companies, hospital or provider data bases, interviews with patients or providers, questionnaires, and diaries. Routine data bases compiled by insurance companies, hospitals, and other health care institutions (such as pharmacies and general practitioners) are commonly used. Insurance companies may seem the most reliable data source. However, they are often unable to provide detailed resource use information due to budget or capitation payed systems. Furthermore, the large multiplicity of providers the back pain patient may see limits the feasibility of the data collection. Even more important is the lack of visibility into medical consumption not covered by the patient's insurance. Moreover, there are no routine records for patients and family resource use, additional expenditures by the patient's or caregiver's household, as well as the input from informal caregivers [3].

Because there are no institutional records tracking information on the type, frequency, and magnitude of these expenditures, data must be obtained through self-reporting channels such as questionnaires and diaries 2, 3. This means that studies undertaking a cost-effectiveness analysis for chronic musculoskeletal pain and from the societal perspective must, at least in part, depend upon information provided by the respondents.

Compared to questionnaires, diaries have been reported to offer several advantages in terms of feasibility and validity 4, 5, 6, 7, 8. While questionnaires usually rely on momentary recall, diaries provide information prospectively over a period of time, resulting in a minimum recall error (forgetting an event entirely, or remembering an event but forgetting its correct date), and therefore generally in better and more complete reporting of the data 4, 5, 6, 9. However, little research has been carried out on the validity of self-reported health care utilization, primarily because of the scarcity of independent “true” data sources to compare self-reports against [10]. If valid costing and utilization data are to be collected, either the time period should be reduced to a limited period or a more accurate data collection method, such as the health diary, should be used 11, 12. However, a survey recall would in general preclude the study of chronic disease care, because a large number of chronic disease therapies are only effective when administered over a longer period of time or complications of the therapies arise only later.

Because no standardized method to estimate total resource use, expenses, and lost production due to illness and treatment appears to be available, we have developed a cost diary. This cost diary allows for the estimation of direct and indirect costs in long-term clinical trials or follow-up studies perceived from a societal perspective. In the present study we examine the feasibility and validity of the cost diary. This article presents the following issues: (1) the development and content of the cost diary; (2) the feasibility and data collection period of the cost diary in two clinical trials; and (3) an aspect of construct validity of the cost diary.

Section snippets

Study samples

The data we use were obtained in two randomized clinical trials evaluating the cost-effectiveness of cognitive behavioral treatment for fibromyalgia (n = 72) and chronic low back pain (n = 133). Patients were enlisted between January 1992 and January 1994. The cost diary has been used in studies for estimating direct health care costs (focusing on pain-related health care utilization), costs incurred by the patient and the family, and indirect costs. Because the focus of this article is on the

Feasibility

Of the chronic low back pain and fibromyalgia patients, 85% completed at least one diary booklet (4 weeks). In total 68% of the diaries were returned, which corresponds to an average of 32 weeks in 1 year. Fig. 2 shows the frequency distribution of the number of weeks the diaries were completed. Fifty percent (103) of the respondents completed the diary for the entire year of follow-up. Fifteen percent did not return any diary, primarily because they withdrew from the study. Results from the

Discussion

In order to perform comprehensive cost-effectiveness studies evaluating treatment for chronic musculoskeletal pain from a societal perspective, it is essential to at least partially obtain information from patients when collecting costing data. In this article we have described the use of a patient diary developed to collect costing data in long-term clinical trials. We have examined the feasibility, data collection period, and aspects of construct validity of the patient cost diary using data

Acknowledgements

The authors gratefully acknowledge Silvia Evers and Steven Linton for their valuable comments on earlier drafts of this article and thank Jolanda van Haastregt, Gerard van Breukelen, Ingrid Nooyen, and Jan van Emmerink for their statistical advice. We also thank Eric Debougnoux (CZ-ziektekostenverzekering) for providing data. The study was supported by a grant from the investigative medicine program of the Dutch Health Insurance Executive Board.

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