Study design
EHES-LUX is a population-based survey performed from 2013 to 2015 that aims to establish baseline information on the general health status of the Luxembourg population aged 25 to 64 years (as the minimum selected age range defined in EHES recommendations [9]). Individuals who met the inclusion criteria filled in a questionnaire. Physical measurements and biological samples were also performed.
Inclusion criteria
Four inclusion criteria were applied in this study: 1) be included in the national health insurance registry, 2) be resident in Luxembourg, 3) be 25–64 years old, and 4) sign an informed consent form. People living in institutions (e.g. hospital, nursing home, prison) were not included in this study.
Sampling design and calculation
Given the small size of the country, a one-stage sampling procedure was performed. In order to obtain a representative sample of the population, a random sample of residents of Luxembourg was defined stratified by age (25–34, 35–44, 45–54, 55–64 years old), gender (male and female) and district of residence (Luxembourg, Diekirch and Grevenmacher). The representative random sample was drawn from the national health insurance registry by the General Inspectorate of Social Security (IGSS). Despite its relevance in the stratification, nationality or country of birth was not provided by IGSS. With a 95% social coverage, this registry is considered the most complete list of inhabitants available in Luxembourg. The 5% remaining people are identified as being mainly the employees of the European Commission (around 12,000) who have their own health insurance system and also asylum seekers (around 1000 new individuals each year) who just arrived and among which almost half are on the way to be registered.
To reach the adequate power and precision to perform age-gender subgroup comparisons, and to observe a 5% prevalence of a specific condition in that subpopulation [1], a minimum of 130 participants were fixed for each age-gender subgroup. Based on this information and on the last Luxembourg census data in 2011 [10], the total required sample size was 1490 participants. Since the minimal participation rate in previous EHES studies was 25% [2] and given the general declining trend in participation rates over time in comparable surveys [11], the participation rate in this present study was assumed to be 23% corresponding to an estimation of the total number of participants to be invited of 6475. Additional file 1: Table S1 illustrates the projection of sample size calculation of each age-sex domain based on the 2011 census.
Recruitment
A letter signed by the Ministry of Health and the Luxembourg Institute of Health (LIH), which realised the study, was sent to the randomly selected individuals in order to inform and invite them to participate in the survey. Along with the letter, an information booklet on the survey, a response card and a pre-paid envelope were also sent. Individuals who were interested in participating in the study were asked to send back the reply coupon enclosed. If no response was received after 3 weeks, non-respondents were contacted again by post. Individuals did not receive any financial incentive for participating in the study.
Those who should be agreed to participate were contacted to fix an appointment for the examination and the collection of biological samples. To facilitate the participants’ access to the study, three different survey sites were available in the country: the Luxembourg Institute of Health (in the middle of the country), the Centre Pontalize (in the North) and the Centre Hospitalier Emile Mayrisch (in the South). We decided to plan two visits despite the possible inconveniences (loss of follow-up), because the completion of the questionnaire and the examination was too long.
Study visit 1
At the examination centre, visit 1 included the completion of a self-administered questionnaire, a medical examination, and the collection of a hair sample. On average, the visit had a duration of approximately 2 h.
The questionnaire included information on the participant’s socioeconomic characteristics, health status (e.g. general health status, diseases and chronic conditions, accidents and injuries, absence from work (due to health problems), physical and sensory functional limitations, personal care activities, household activities, pain, mental well-being and sleep disorders), health care (e.g. use of inpatient and day care, use of ambulatory and home care, medicine use, preventive services, unmet needs for health care), health determinants (e.g. self-reported height and weight, physical activities / exercise, consumption of fruit and vegetables, social support, provision of informal care or assistance, tobacco use, alcohol consumption and drugs) and women’s health.
Interviews and medical examinations were conducted by trained clinical research nurses in English, German, French or Portuguese. Questionnaires were language-validated [12]. The validation process consisted in three steps: 1) the English version was translated to German, French or Portuguese by a certified translator whose the mother tongue was the target language; 2) the first version was back-translated to English by an English native speaker and compared to the original version; and 3) based on the discrepancies between the back-translated and the original versions, an improved version in German, French and Portuguese was created.
Examinations included blood pressure and anthropometric measurements, spirometry, visual acuity, thyroid function and an electrocardiogram.
Hair samples from each participant were obtained and stored at the Human Biomonitoring Research Unit of the LIH in order to measure pollution and pesticide consumption indicators in future research.
Study visit 2
A second visit was held at the laboratory where blood and urine samples were taken and analysed from each participant. Blood samples were used to measure plasma glucose, glycated haemoglobin (HbA1c), triglycerides, total and HDL cholesterol (LDL cholesterol calculated), creatinine, and thyroid function (TSH, anti-TPO). Urine samples were obtained to measure ioduria, creatinuria and microalbuminuria.
Results of medical examinations from visit 1 and biological analyses from visit 2 were evaluated and validated by a clinical committee (composed of a cardiologist, a pulmonologist and a diabetologist) who verified the presence or absence of anomalies.
Eligibility status
Individuals were classified as: 1) eligible, if she/he met the inclusion criteria, 2) not eligible if she/he did not met the inclusion criteria or 3) unresolved (e.g. if the invitation letter was returned due to change of address, contacts were not possible or not successful, no information was available to assess the eligibility status). An eligible person was classified as participant if she/he had at least one valid examination measurement (e.g. height and weight) in addition to the completed questionnaire. If the individual contacted agreed to participate, it was recorded as a positive answer. If the eligible person refused to participate she/he was classified as non-participant.
Definition of variables
From participant’s height (in meters) and weight (in kilograms), Body mass index (BMI) was calculated as the ratio of weight divided by the square of height. Self-reported and measured BMI were recorded. Subjective BMI was calculated based on participant’s self-reported value and measured BMI was calculated from the value measured by clinical research nurses. We categorised BMI as normal weight (< 25 kg/m2), overweight (25 to < 30 kg/m2) and obese (≥30 kg/m2).
Participants’ blood pressure was measured 3 times on the right arm in a sitting position [13]. Blood pressure was measured after a 5-min rest and with 1-min intervals between the following two measurements. Both systolic and diastolic pressure was calculated as the mean of his/her second and third measurements. Measured hypertension was defined as systolic/diastolic blood pressure greater than or equal to 140/90 mmHg. Self-reported hypertension was defined as hypertension being diagnosed by a physician. Information on medications for high blood pressure was obtained from the questions “In the past 2 weeks, have you used any medicines that were prescribed to you by a doctor? Were these medications for high blood pressure?”
Measured diabetes was defined as fasting plasma glucose higher than or equal to 126 mg/dL. Self-reported diabetes was defined as being diagnosed as diabetic by a physician. Information on glucose lowering medications was obtained from the questions “In the past 2 weeks, have you used any medicines that were prescribed to you by a doctor? Were these medications for diabetes?”
Hypercholesterolemia was defined as total cholesterol values higher than or equal to 200 mg/dL. Self-reported hypercholesterolemia was defined as being diagnosed by a physician as having high cholesterol. Information on blood cholesterol lowering medications was obtained from the questions “In the past 2 weeks, have you used any medicines that were prescribed to you by a doctor? Were these medications for lowering blood cholesterol levels?”
Depression was evaluated with the Patient Health Questionnaire (PHQ-9) defined by Kroenke et al. [14], and completed by participants. We defined depressive symptoms as a PHQ-9 score of greater than or equal to 10 (the maximum score is 27). Self-reported depression was defined as being diagnosed by a physician as having depression. Information on anti-depressive medications was obtained from the questions “In the past 2 weeks, have you used any medicines that were prescribed to you by a doctor? Were these medications for depression?”
Privacy of participants
The study participants’ data privacy (confidentiality, integrity and availability) was maintained throughout the entire study. Each participant was assigned a participant number. Identifiable paper or electronic source documents (e.g. signed informed consent forms, medical feedback letter and appointment books) were kept in a secure location under the responsibility of the medical investigator. We used Ennov Clinical® as the data management system and defined data access rights to each survey member (data entry clerk, data manager, statistician, project manager, etc) according to their responsibilities. Moreover, any data entry / modification was automatically logged to a non-changeable audit trail.
Statistical analysis
For each age and gender stratum, sampling weights were used to account for differences between participants and non-participants (See details in Additional file 2: Table S2). Moreover, as Luxembourg is a multicultural country, a comparison between participants and Luxembourg population according to the country of birth was performed. Weights were calculated from the selection probabilities (with 2011 Luxembourg census data as a reference) and were adjusted for non-response.
Response rates were defined as the ratio of the number of positive answers over the number of invitations sent. Participation rates were defined as the ratio of the number of participants over the number of eligible and unresolved individuals [15].
The representativeness of the sample and the comparability of participants and non-participants were analysed with a one-way ANOVA test. Gender and district effects on the number of study participants were estimated by a Poisson regression analysis.
Stratification and all statistical analyses were carried out using SAS version 9.4 (SAS Institute Inc., Cary, NC, USA). A two-sided P < 0.05 was considered statistically significant.
