Agreement between self-reported data on medicine use and prescription records vary according to method of analysis and therapeutic group

Abstract

Objective

This study compared national self-reported data on medicine use and national prescription records at the individual level.

Study Design and Setting

Data from the nationally representative Danish health survey conducted in 2000 (n = 16,688) were linked at the individual level to national prescription records covering 1999–2000. Kappa statistics and 95% confidence intervals were calculated.

Results

Applying the legend time method to medicine groups used mainly on a chronic basis revealed good to very good agreement between the two data sources, whereas medicines used as needed showed fair to moderate agreement. When a fixed-time window was applied for analysis, agreement was unchanged for medicines used mainly on a chronic basis, whereas agreement increased somewhat compared to the legend time method when analyzing medicines used as needed.

Conclusion

Agreement between national self-reported data and national prescription records differed according to method of analysis and therapeutic group. A fixed-time window is an appropriate method of analysis for most therapeutic groups.

Introduction

Prescription records and self-reported data from surveys are often used to measure various aspects of the population's medicine use patterns at the individual level [1], [2]. However, little is known about the agreement of these two data sources: which is preferable, and in what situations?

Most prescription records have the advantage of being administered electronically and thus accuracy is high [3], [4]. Prescriptions can be collected at the general practitioner level or at the pharmacy level. If the register has full cover, the pharmacy level will be more accurate [4], as not all medicine prescribed by a general practitioner is purchased at the pharmacy, and further, not all medicines dispensed are actually used. Reviews have reported that adherence among patients suffering from chronic diseases such as hypertension, depression, and asthma averages only 50% [5], [6]. Prescription records do not include information on purchase of over-the-counter (OTC) medicine, and information on individuals is usually sparse.

Due to the high accuracy, prescription records are usually seen as the gold standard compared with self-reported medicine use [4].

Self-reported data provide information on which medicines are actually used, but self-reporting is prone to various forms of bias (e.g., recall bias) and respondents may be reluctant to report sensitive data such as medicine use [7], [8]. Surveys have the advantage of possible inclusion of extensive information on the respondent such as use of OTC medicines, herbal medicines, and other types of health care services as well as disease and sociodemography.

Few large, population-based studies dealing with the correlation of registers and self-reported data are reported in international literature. Most studies are rather small and often conducted in specific populations such as participants in case-control studies or specific patient and age groups [9], [10], [11]. We have identified only two large, population-based studies comparing self-reported data on medicine use and pharmacy records [12], [13].

In a population-based Dutch cohort (n = 7,568), sensitivity of pharmacy records was calculated with a questionnaire as the gold standard and using a 1-year fixed-time window [12]. Good sensitivity was found for antihypertensives, lipid lowering drugs, oral contraceptives, hormone replacement therapy (HRT), and oral antidiabetics, whereas nitrates and OTC painkillers had low sensitivity [12]. A population-based American study (n = 1,430) found that the congruence between self-reported medicine use and pharmacy records was high at the overall level using a legend time method analyzing the pharmacy records [13]. Congruence was better for medicines used for serious conditions or on a regular basis [13].

The Danish Register of Medicinal Product Statistics provides a unique opportunity to study individual medicine use at the national level, as the register covers all Danish pharmacies and thus captures all prescriptions purchased by every resident of the country in a Danish pharmacy.

The aim of this study was to analyze the agreement of data on medicine use at the individual level by comparing national self-reported data and national prescription records.

No study has previously applied national representative data when dealing with this topic.