Lau Caspar Thygesen

Introduction to Danish (nationwide) registers on health and social issues: Structure, access, legislation, and archiving

Danish registers contain information on many important health and social issues. Because all Danish citizens have a unique personal identification number, linkage at the individual level between these nationwide registers and other data sources is possible and feasible. In this paper we briefly introduce selected Danish registers and the data structure and requirements for getting access to data at Statistics Denmark, which is the main provider of register data. We introduce the Danish Data Archive and briefly present the Act on Processing of Personal Data, which is the legal foundation for analyses of register-based data in Denmark.

Danish registers on personal labour market affiliation

Introduction: Statistics Denmark has a number of registers about labour market affiliation, which includes information on employees, establishments and the relation between employees and establishments. Content: The registers describe a person’s attachment to the labour market, information on the establishments and information on the relations between persons and establishments. The registers presented either include information at one specific point in time each year (end of November) or during the year. Validity and coverage: Both the validity and coverage of the registers are considered to be high. Conclusion: These registers provide valuable information on personal labour market affiliation and can be used to study determinants and consequences of labour market affiliation.

When the entire population is the sample: strengths and limitations in register-based epidemiology

Studies based on databases, medical records and registers are used extensively today in epidemiological research. Despite the increasing use, no developed methodological literature on use and evaluation of population-based registers is available, even though data collection in register-based studies differs from researcher-collected data, all persons in a population are available and traditional statistical analyses focusing on sampling error as the main source of uncertainty may not be relevant. We present the main strengths and limitations of register-based studies, biases especially important in register-based studies and methods for evaluating completeness and validity of registers. The main strengths are that data already exist and valuable time has passed, complete study populations minimizing selection bias and independently collected data. Main limitations are that necessary information may be unavailable, data collection is not done by the researcher, confounder information is lacking, missing information on data quality, truncation at start of follow-up making it difficult to differentiate between prevalent and incident cases and the risk of data dredging. We conclude that epidemiological studies with inclusion of all persons in a population followed for decades available relatively fast are important data sources for modern epidemiology, but it is important to acknowledge the data limitations.

Danish population-based registers for public health and health-related welfare research: introduction to the supplement

Denmark and other Nordic countries have exceptional opportunities to perform register-based research, because of the unique personal identification number available to all persons with permanent residence [1]. This number makes it possible to link information at the individual level from several registers for investigation of various research questions. The unique personal identification number was introduced in Denmark in 1968, which enables follow-up of individuals for decades. This supplement of the Scandinavian Journal of Public Health presents public health and health-related registers and health-related welfare research based on Danish nationwide registers. The topics range from presentations of important registers, to introduction to the Danish legal foundations for register-based research, to short reviews of selected applications of registers for public health research. Linkage of the Danish registers is based on three base registers including identification numbers for persons, businesses, and real estates (Figure 1). All three base registers have linking keys to related registers and also to the other base registers. The linking keys are: the personal identification number (CPR-number) [1], the business identification number (SE/CVR-number) [2], and the building and housing identification number (BBR-number) [3]. The Civil Registration System contains the CPRnumber and includes references to parents and spouses, making it possible to establish the family unit. The Business Register contains the SE/CVRnumber of all businesses. The Building and Housing Register (BBR) contains the BBR-number, which identifies all unique housing unities. By the base registers it is possible to link persons, businesses and housing units. The aim of the supplement is to present a wide range of Danish registers and register-based research. We invited Danish key researchers performing register-based research to contribute to this supplement. Thus, this publication also highlights that registerbased research in Denmark is widely distributed at various research institutions and that registers are fundamental data sources in health and healthrelated welfare research. Twenty two institutes and departments at four Danish universities and 21 Danish administrative and research institutions contributed to the papers in this supplement. The supplement has three sections. The first section gives an overview of Danish registers, briefly introduces how to get access to data at Statistics Denmark and the legal foundation for register-based research. The section also presents an online database, which gives further information and an overview of the content of various Danish registers. This database also includes a search engine to help researchers to get more details on register contents. The second section includes presentations of important Danish registers on health and social factors. All papers have the same format, including four sections – in the Introduction section the background and rationale for establishing the register, the current purpose and historical pioneers are presented. In the Content section information on central variables in the register is provided and changes in variables and content, in reporting to the

Modifiable risk factors and survival in women diagnosed with primary breast cancer: results from a prospective cohort study.

This study examines the impact of smoking, body mass index, alcohol consumption, hormone replacement therapy, and physical activity on all-cause mortality among 528 Danish women diagnosed with primary breast cancer. Participants were women enrolled in the Copenhagen City Heart Study. Prospective self-reported exposure information was collected from four points of follow-up in 1976–1978, 1981–1983, 1991–1994, and 2001–2003. Kaplan–Meier survival curves and multivariate Cox regression analyses were performed adjusting for age, disease stage, adjuvant treatment, menopausal status, parity, alcohol intake, smoking, physical activity, body mass index, and hormone replacement therapy. The study shows that smoking for total mortality [hazard ratio, 1.16; 95% confidence interval, 1.05–1.29] and obesity for both total mortality (1.61; 1.12–2.33) and breast cancer-specific mortality (1.82; 1.11–2.99) were significantly associated with decreased survival after breast cancer diagnosis. A moderate alcohol intake of 1-6 units/week (0.85; 0.64–1.12), 7–14 units/week (0.77; 0.56–1.08), and treatment with hormone replacement therapy (0.79; 0.59–1.05) were less than 1, but not statistically significantly associated with prolonged survival. A moderate physical activity of 2–4 h/week (1.07; 0.77–1.49) and a high physical activity of more than 4 h/week (1.00; 0.69–1.45) showed no association with survival after breast cancer diagnosis.

Time trends in physical activity in leisure time in the Danish population from 1987 to 2005

Aims: To examine time trends in leisure-time physical activity in the Danish population from 1987 to 2005. Methods: Analyses were based on four national representative population surveys of 4752 respondents in 1987, 4667 in 1994, 16,688 in 2000, and 14,566 in 2005. Associations between leisure-time physical activity (dichotomised into high versus low) and temporal changes according to year of birth, age, and calendar-year were estimated in an age-period-cohort logistic regression model. Time trends in socioeconomic differences in physical activity were analysed by testing interaction terms. Results: The odds ratio (OR) of being highly physically active in leisure time in 2005 compared to 1987 was 1.6 (95% CI: 1.3—1.9) among men and 1.8 (95% CI: 1.5—2.2) among women. The age- and calendar-year-adjusted probability of being highly physically active increased with year of birth. The OR of being highly physically active among men and women with 13 or more years of education were 2.1 (95% CI: 1.8—2.3) and 2.0 (95% CI: 1.7—2.2), respectively, compared to individuals with less than 10 years of education. There was no significant interaction between educational level and year of interview (p>0.05). Conclusions: During the past 20 years, physical activity in leisure time has increased. Younger generations seem to be more physically active in leisure time than older generations. Due to the stable socioeconomic inequality gap in physical activity, future intervention strategies should target these inequalities.

Cumulative years in occupation and the risk of hip or knee osteoarthritis in men and women: a register-based follow-up study

Objectives Occupational workload has been associated with an increased risk of osteoarthritis (OA), but only little research has been conducted among female workers. The objective of this study was to analyse if men and women in farming, construction or healthcare work have increased risk of developing OA of the hip or knee. Methods A follow-up study based on register data of the whole Danish working population in the period 1981 to 2006 followed up for hip or knee OA during 1996 to 2006. Cumulative years in occupation were calculated for assessment of dose–response relationship. Gender-specific analyses were carried out with Cox regression models using age as timescale and adjusting for calendar period, income, unemployment and previous knee injury, and done separately for hip and knee OA. Results Male floor layers and bricklayers and male and female healthcare assistants had the highest risks of knee OA, and farmers had the highest risk of hip OA. Male farmers had increased risk of hip OA already after 1–5 years in occupation (HR, 1.63) and a dose–response-related risk of hip OA (HR up to 4.22). Generally, the risk of OA increased with cumulative years in the occupation in both men and women. Conclusions Occupations with heavy physical workload present a strong risk for hip and knee OA in both men and women, and the risks increase with cumulative years in occupation and noticeable hip OA among male farmers.

Effects of sample attrition in a longitudinal study of the association between alcohol intake and all‐cause mortality

BACKGROUND AND AIMS Longitudinal studies show higher mortality among abstainers and heavy drinkers than among light and moderate alcohol consumers. The influence on this association of missing information on alcohol intake due to attrition (dropout) has not been examined previously. The aims of this study were to characterize participants who dropped out and to evaluate whether the missing information influenced the association between alcohol intake and all-cause mortality. DESIGN AND PARTICIPANTS Data on the 18 974 participants in the Copenhagen City Heart Study, with four measures of alcohol intake and other life-style factors during 28 years of follow-up, were linked with nation-wide registers on socio-economic covariates, mortality and disease incidence. Logistic regression was used to describe life-style and socio-economic determinants of attrition, and Poisson regression was used to evaluate how attrition affected the association between alcohol intake and mortality. The statistical methods used for dealing with missing values were complete case analysis, carry last observation forward, simple imputations, multiple imputation and weighting. FINDINGS Abstinence and high alcohol intake, current smoking, physical inactivity and high body mass index increased the odds of dropping out, whereas being married, more years of education, skilled occupation, high income and large residential area decreased the odds. Attrition was associated with increased mortality and incidence rates of heart disease, lung and upper digestive tract cancers and alcoholic liver diseases. Increased mortality among abstainers and heavy drinkers was observed with all methods used for handling missing data on alcohol intake. CONCLUSIONS Attrition was non-random, and the observed association between alcohol intake and all-cause mortality did not differ by statistical method for handling missing data.

Evaluating the use and limitations of the Danish National Patient Register in register-based research using an example of multiple sclerosis

Mason K, Thygesen LC, Stenager E, Brønnum‐Hansen H, Koch‐Henriksen N. Evaluating the use and limitations of the Danish National Patient Register in register‐based research using an example of multiple sclerosis. 
Acta Neurol Scand: 2012: 125: 213–217. 
© 2011 John Wiley & Sons A/S.

Alcohol intake and colorectal cancer: a comparison of approaches for including repeated measures of alcohol consumption.

Background: In numerous studies, alcohol intake has been found to be positively associated with colorectal cancer risk. However, the majority of studies included only one exposure measurement, which may bias the results if long-term intake is relevant. Methods: We compared different approaches for including repeated measures of alcohol intake among 47,432 US men enrolled in the Health Professionals Follow-up Study. Questionnaires including questions on alcohol intake had been completed in 1986, 1990, 1994, and 1998. The outcome was incident colorectal cancer during follow-up from 1986 to 2002. Results: During follow-up, 868 members of the cohort experienced colorectal cancer. Baseline, updated, and cumulative average alcohol intakes were positively associated with colorectal cancer, with only minor differences among the approaches. These results support moderately increased risk for intake >30 g/d and weaker increased risk for lower intake. The hazard ratio for baseline alcohol intake was 1.07 (95% confidence interval = 1.02–1.11) per 10 g/d increase, which was similar for updated and cumulative average alcohol intake. Consistent moderate and high alcohol intake showed increased risk, and the relative risk decreased slightly with longer latency time. Alcohol frequency was positively associated with cancer risk among men with alcohol intake above 15 g/d. Conclusions: Alcohol intake was positively associated with colorectal cancer, with minor differences among analytic approaches (which may be attributable to low intraindividual variation during follow-up).