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The primary function of the kidney is the production of urine through filtration of blood. The entire blood volume passes through our kidneys about 40 times a day during which toxins are removed, and the concentrations of minerals such as sodium, potassium, calcium, and phosphate are held constant. Toxins and other waste products leave the body through excretion into the urine produced by the kidneys. The Glomerular Filtration Rate (GFR) refers to the amount of blood filtered through the kidney in a certain amount of time, usually expressed as ml/min. The GFR is considered worldwide the gold standard indicator for kidney function.

With increasing age, kidneys become less able to thoroughly filter blood to retain important blood constituents (e.g. Albumin) and eliminate toxins. Impaired kidney function can be diagnosed through detection of elevated levels of urinary substances (e.g. Creatinine or Cystatin C) in the blood. The presence of Albumin in urine (i.e. Albuminuria) is a sign for impaired filtration function of the kidneys. 

It is essential that physicians estimate kidney function in older individuals as accurately as possible to adapt the doses of prescribed renally excreted drugs which could potentially be damaging to the kidneys. This could be done by way of a somewhat time-consuming measurement of GFR through the administration of a contrast medium and the repeated measurement of its excretion. In everyday clinical work, mathematical evaluation equations which do not require the use of contrast media are usually used. Valid equations for the evaluation of kidney function in individuals aged 70 and older were unfortunately lacking before the initiation of the BIS. Hence, one of the main goals of the BIS was to develop valid equations to estimate GFR in the elderly population.

Study Design

The BIS is a population-based cohort study which was conceptualized in 2008 and 2009 and piloted at the end of 2009. Data collection for the main study began in January 2010. The study followed two approaches:

  • Cross-sectional approach:
    By means of random sampling in cooperation with the statutory insurance company “Allgemeine Ortskrankenkasse (AOK) Nordost”, 2069 (target: 2000) AOK-insured individuals were included in the study. During face-to-face visits, information about demographics, comorbidities, and various lifestyle variables were gathered using a computer-based questionnaire complemented by a physical examination to obtain anthropometric data. Furthermore, blood and urine samples were collected from participants to determine the prevalence of impaired kidney function as well as other associated risk factors in old age.

    A subset of the study population (N = 600) underwent an additional one-time procedure to measure their GFR using the contrast medium “Iohexol”. This procedure was conducted in order to develop two equations (BIS1 and BIS2) for the estimation of GFR specifically for individuals aged 70 and older.
  • Longitudinal (prospective) cohort approach:
    Over the course of ten years, the study visits described above including the collection of “Biosamples” in all 2069 participants were conducted biennially to assess the progression of kidney function from the age of 70 onwards. Moreover, the frailty status of participants was recorded as well as other comorbidities such as cardiovascular diseases, and death.

Methodological Challenges

Individuals aged 70 and older are generally underrepresented in epidemiological studies. This also holds true when investigating the long term course of kidney function in old age. This could be attributed -in part- to the following inherent challenges:

  • The recruitment and retention of elderly participants in longitudinal studies over years are difficult.
  • Data collection from elderly participants is time-consuming due to age-related concomitant illnesses, vision and/or hearing loss, cognitive impairment, and frailty.

Furthermore, the involvement of older participants highlighted some of their psychosocial needs that played an important role in maintaining their long term participation in later follow-up rounds of the study. Recognition of such needs by the BIS team members and their exceptional dedication largely contributed to the successful retention of the participants over the whole ten-year data collection phase.