The Berlin initiative study: the methodology of exploring kidney function in the elderly by combining a longitudinal and cross-sectional approach.

Epidemiologic data on incidence, prevalence and risk factors for chronic kidney disease (CKD) and its progression to kidney failure in people > or =70 years are scarce. This lack may have two reasons: First, the issue has only recently gained importance by the changing demographics characterized by an aging society. Secondly, a validated method for estimating kidney function in terms of glomerular filtration rate (GFR) in the elderly is still lacking. In this paper we describe the methodology of a combined longitudinal and cross-sectional approach of a population based study which will start in January 2010. The aims of the study are to identify prevalent and incident cases of CKD as well as co-morbidities and associated risk factors for progression of disease in this specific age-group. To assess prevalence, a new GFR estimation equation is to be developed. In a longitudinal approach a population based, age stratified sample of 2,000 subjects > or =70 years will be randomly drawn from a data base of a large health insurance company. Interview, physical examination, and preliminary estimation of GFR, based on serum creatinine will be performed. The entire cohort will be followed over the course of 2 years. In a cross-sectional approach a subsample of 600 subjects will be defined based on preliminary GFR values. Kidney function will be determined by measuring plasma clearance of an exogenous filtration marker (Iohexol). A new GFR-equation will be developed and validated using Iohexol clearance as gold standard to estimate GFR accurately and precisely. Data of 2,000 subjects will be used to estimate prevalence of CKD.