Improved understanding of sex and gender-specific differences in the aetiology, mechanisms and epidemiology of chronic kidney disease (CKD) could help nephrologists better address the needs of their patients. Population-based studies indicate that CKD epidemiology differs by sex, affecting more women than men, especially with regard to stage G3 CKD. The effects of longer life expectancy on the natural decline of glomerular filtration rate (GFR) with age, as well as potential overdiagnosis of CKD through the inappropriate use of GFR equations, might be in part responsible for the greater prevalence of CKD in women. Somewhat paradoxically, there seems to be a preponderance of men among patients starting renal replacement therapy (RRT); the protective effects of oestrogens in women and/or the damaging effects of testosterone, together with unhealthier lifestyles, might cause kidney function to decline faster in men than in women. Additionally, elderly women seem to be more inclined to choose conservative care instead of RRT. Dissimilarities between the sexes are also apparent in the outcomes of CKD. In patients with predialysis CKD, mortality is higher in men than women; however, this difference disappears for patients on RRT. Although access to living donor kidneys among men and women seems equal, women have reduced access to deceased donor transplantation. Lastly, health-related quality of life while on RRT is poorer in women than men, and women report a higher burden of symptoms. These findings provide insights into differences in the underlying pathophysiology of disease as well as societal factors that can be addressed to reduce disparities in access to care and outcomes for patients with CKD.

The aim of this study was to investigate 28-day mortality after COVID-19 diagnosis in the European kidney replacement therapy population. In addition, we determined the role of patient characteristics, treatment factors, and country on mortality risk with the use of ERA-EDTA Registry data on patients receiving kidney replacement therapy in Europe from February 1, 2020, to April 30, 2020. Additional data on all patients with a diagnosis of COVID-19 were collected from 7 European countries encompassing 4298 patients. COVID-19attributable mortality was calculated using propensity score-matched historic control data and after 28 days of follow-up was 20.0% (95% confidence interval 18.7%-21.4%) in 3285 patients receiving dialysis and 19.9% (17.5%-22.5%) in 1013 recipients of a transplant. We identified differences in COVID-19 mortality across countries, and an increased mortality risk in older patients receiving kidney replacement therapy and male patients receiving dialysis. In recipients of kidney transplants ‡75 years of age, 44.3% (35.7%-53.9%) did not survive COVID-19. Mortality risk was 1.28 (1.02-1.60) times higher in transplant recipients compared with matched dialysis patients. Thus, the pandemic has had a substantial effect on mortality in patients receiving kidney replacement therapy, a highly vulnerable population due to underlying chronic kidney disease and a high prevalence of multimorbidity.

Considerable variation exists in the current demographics of children treated with RRT across Europe.

In clinical epidemiology, experimental studies usually take the form of randomized controlled clinical trials (RCTs). The data analysis of an RCT can be performed by using two complementary strategies, that is according to the intention to treat (ITT) principle and the per protocol (PP) analysis. By using the ITT approach, investigators aim to assess the effect of assigning a drug whereas by adopting the PP analysis, researchers investigate the effect of receiving the assigned treatment, as specified in the protocol. Both ITT and PP analyses are essentially valid but they have different scopes and interpretations dependent on the context.

Despite a widespread preconception that HD should be reserved for cases in which PD is not feasible, in Europe, we found 1 in 8 infants in need of maintenance dialysis to be initiated on HD therapy. Patient characteristics at dialysis therapy initiation, prospective survival, and time to transplantation were very similar for infants initiated on PD or HD therapy.

We aimed to describe survival in European pediatric dialysis patients and compare the differential mortality risk between patients starting on hemodialysis (HD) and peritoneal dialysis (PD). Data for 6473 patients under 19 years of age or younger were extracted from the European Society of Pediatric Nephrology, the European Renal Association, and European Dialysis and Transplant Association Registry for 36 countries for the years 2000 through 2013. Hazard ratios (HRs) were adjusted for age at start of dialysis, sex, primary renal disease, and country. A secondary analysis was performed on a propensity score-matched (PSM) cohort. The overall 5-year survival rate in European children starting on dialysis was 89.5% (95% confidence interval [CI] 87.7%-91.0%). The mortality rate was 28.0 deaths per 1000 patient years overall. This was highest (36.0/1000) during the first year of dialysis and in the 0- to 5-year age group (49.4/1000). Cardiovascular events (18.3%) and infections (17.0%) were the main causes of death. Children selected to start on HD had an increased mortality risk compared with those on PD (adjusted HR 1.39, 95% CI 1.06-1.82, PSM HR 1.46, 95% CI 1.06-2.00), especially during the first year of dialysis (HD/PD adjusted HR 1.70, 95% CI 1.22-2.38, PSM HR 1.79, 95% CI 1.20-2.66), when starting at older than 5 years of age (HD/PD: adjusted HR 1.58, 95% CI 1.03-2.43, PSM HR 1.87, 95% CI 1.17-2.98) and when children have been seen by a nephrologist for only a short time before starting dialysis (HD/PD adjusted HR 6.55, 95% CI 2.35-18.28, PSM HR 2.93, 95% CI 1.04-8.23). Because unmeasured case-mix differences and selection bias may explain the higher mortality risk in the HD population, these results should be interpreted with caution.

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Study quality depends on a number of factors, one of them being internal validity. Such validity can be affected by random and systematic error, the latter also known as bias.Both make it more difficult to assess a correct frequency or the true relationship between exposure and outcome. Where random error can be addressed by increasing the sample size, a systematic error in the design, the conduct or the reporting of a study is more problematic. In this article, we will focus on bias, discuss different types of selection bias (sampling bias, confounding by indication, incidence-prevalence bias, attrition bias, collider stratification bias and publication bias) and information bias (recall bias, interviewer bias, observer bias and lead-time bias), indicate the type of studies where they most frequently occur and provide suggestions for their prevention. K E Y W O R D S bias, epidemiologic methods, research design, research methodologyWhere external validity represents the degree to which results of a study may apply to populations or groups that did not participate in the study, 1