BackgroundSelection bias is a threat to the internal validity of epidemiological studies. In light of a growing number of studies which aim to provide DNA, as well as a considerable number of invitees who declined to participate, we discuss response rates, predictors of lost to follow-up and failure to provide DNA, and the presence of possible selection bias, based on five samples of adolescents.MethodsWe included nearly 7,000 adolescents from two longitudinal studies of 18/19 year olds with two corresponding cross-sectional baseline studies at age 15/16 (10th graders), and one cross-sectional study of 13th graders (18/19 years old). DNA was sampled from the cheek mucosa of 18/19 year olds. Predictors of lost to follow-up and failure to provide DNA were studied by Poisson regression. Selection bias in the follow-up at age 18/19 was estimated through investigation of prevalence ratios (PRs) between selected exposures (physical activity, smoking) and outcome variables (general health, mental distress, externalizing problems) measured at baseline.ResultsOut of 5,750 who participated at age 15/16, we lost 42% at follow-up at age 18/19. The percentage of participants who gave their consent to DNA provision was as high as the percentage that consented to a linkage of data with other health registers and surveys, approximately 90%. Significant predictors of lost to follow-up and failure to provide DNA samples in the present genetic epidemiological study were: male gender; non-western ethnicity; postal survey compared with school-based; low educational plans; low education and income of father; low perceived family economy; unmarried parents; poor self-reported health; externalized symptoms and smoking, with some differences in subgroups of ethnicity and gender. The association measures (PRs) were quite similar among participants and all invitees, with some minor discrepancies in subgroups of non-western boys and girls.ConclusionsLost to follow-up had marginal impact on the estimated prevalence ratios. It is not likely that the invitation to provide DNA influenced the response rates of 18/19 year olds. Non-western ethnicity, male gender and characteristics related to a low social class and general and mental health problems measured at baseline are associated with lost to follow-up and failure to provide DNA.

BackgroundBullying is a widespread and serious problem that might influence both mental and psychical well being as well as school performance and social life. The aim of this study was to describe the prevalence of bullying, mental health problems and psychical complaints among 10th and 12th grade students and to analyze the association between bullying, mental health problems and muscle and skeletal complaints.MethodsTwo cross sectional studies of adolescents living in Oslo, Norway the first conducted in 2001 among 10th grade students (15/16 years old) and the second in 2004 among 12th grade students (18/19 years old). Both surveys were based on self report, were mostly school based and had almost identical questionnaires. There were around 3700 participants in both surveys, but the participation rate was lower in the latter survey (88 versus 80%). The Hopkins Symptoms Check List (HSCL-10) and the Strength and Difficulties Questionnaire (SDQ) were used to measure mental health problems.ResultsBullying is decreasing both among boys and girls while the prevalence of internalized mental health problems are increasing from 10th to 12th grade. For muscle and skeletal pain there is a diverging trend between boys and girls, with an increase among girls and a decrease among boys. The highest Odds Ratios, as a measure for the association between bullying, mental health problems and pain, were found for internalized mental health problems at both 10th and 12th grade both for boys and girls.ConclusionBoth internalized and externalized mental health problems together with pain seem to be associated with bullying irrespective of school type and gender.

The main aim of this study was to investigate the effect of pain complaints at baseline and mental distress at follow-up. We included adolescents from two longitudinal studies of 18/19 years of age with two corresponding cross-sectional baseline studies at age 15/16. A total of 5750 were invited for the baseline study, and we have 3-year follow-up data for 3316 (57.8% follow-up rate). All information is based on self-report. The questions of pain are asking for severe pain the past year. Mental distress is measured by Hopkins Symptoms Check List (HSCL-10). Number of pain sites increased among girls and decreased among boys from baseline to follow-up. There was a cross-sectional dose-response association between number of pain sites and mental distress at both time points. Also, in the follow-up study, there was dose-response relationship between numbers of pain sites at baseline and mental distress at follow-up even after controlling for possible confounding factors. We found no effect of mental distress at baseline on pain reports at follow-up. There is a strong association between pain at 15/16 years and mental distress at 18/19 years of age. Clinicians should therefore take pain complaints among adolescents seriously and be aware of comorbid and later development of mental health problems.

The aim of this study was to test the hypothesis that atopic conditions at 15/16 years of age affect both internalized and externalized mental health problems 3 years later. Combined school and postal survey was conducted in urban and rural settings. A total of 3,674 adolescents (70.1% response rate) were followed at two time points and interviewed with similar questionnaires at baseline and follow-up. Hopkins Symptoms Checklist (HSCL-10) was used to assess internalized problems, and two subscales (conduct problems and hyperactivity) from the Strength and Difficulties Questionnaire to measure externalized mental health problems. The atopic conditions investigated were asthma, hay fever and eczema by asking the adolescents whether these conditions were present or not. There was an increase in the prevalence of internalized mental health problems from about 17-25% and a decrease in externalized mental health problems and number of atopic conditions in the follow-up period. Of the atopic conditions, hay fever was most prevalent with about 34% at 15 years of age and 20% at 18. The asthma prevalence was at 10 and 5% and eczema at 25 and 10%, respectively. Internalized mental health problems among girls were significantly associated with atopic conditions 3 years earlier, also after controlling for confounding variables. To live with atopic conditions seem to affect the mood and level of anxiety among adolescent girls. This should be kept in mind by health professionals treating young girls with atopic conditions.

Some epidemiological evidence shows a link between abnormality of lipid profiles and variations in serum calcium. However, it is unknown whether this association resulted from confounding factors. The present study was designed to investigate the relationship between serum lipids and calcium. Serum calcium was corrected for albumin. Major confounding factors including age, gender, medications, menopause, parathyroid hormone (PTH) and 25-OH-vitamin D status were controlled in analyses. A total of 1907 adult subjects from the province of Newfoundland and Labrador (NL), Canada participated in the study. Significant positive correlations were detected between serum total cholesterol and high density lipoprotein-cholesterol (HDL-c) with variations of serum Ca++ in both genders (p < 0.05–0.0001). Significant positive correlations were additionally detected between triglycerides (TG) and low density lipoprotein-cholesterol (LDL-c) with Ca++ in women only (p < 0.0001) in partial correlation analyses. Similar significant results were detected in both females and males not taking any medication. Analyses were performed based on menopausal status as well. Significant correlations were seen in both pre- and post-menopausal women but higher correlation coefficients were observed in pre-menopausal women as compared to post-menopausal women. Subjects with low calcium levels had the lowest concentration of total cholesterol, TG, HDL-c and LDL-c, while subjects with high calcium levels had the highest concentration of all four markers in women. The significant associations between cholesterol, TG and LDL-c and serum Ca++ remained after calcium was adjusted for 25-OH-vitamin D and PTH. Our results indicate that the abnormality of serum lipid profiles are significantly correlated with altered serum Ca++ levels independent of age, obesity status, medication, phosphorus, magnesium, 25-OH-vitamin D and PTH.