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and that nearly one-third of these subjects had stable or even increasing Ccr with aging. However, this study had several limitations. The health status of the subjects was poorly described and an unknown number had type 2 diabetes, a disorder known to have a marked influence on GFR, even in the absence of proteinuria [10] . Moreover, GFR itself was not measured and 24 h endogenous Ccr is no longer recommended to estimate GFR because of lack of precision and high intra-individual, between test variation [11] . This last point is particularly relevant for longitudinal studies [12] , as statistically significant slopes for a change of GFR or Ccr require many points of observation. Too few points were available in many patients in the Baltimore Longitudinal Study of Aging. The study by Malmgren et al. [13] in the current issue of Nephron can be considered as partially remedying some of the deficiencies of earlier studies. Indeed, the authors followed eGFR, but not mGFR, using several established serum creatinine-based estimating equations in a large cohort of elderly Swedish women longitudinally for 10 years. They showed interesting results concerning the prevalence of chronic kidney disease (CKD) in the elderly, demonstrated the average decline of eGFR over time in the elderly and, lastly, examined the association beThe conjecture that measured glomerular filtration rate (mGFR) declines with aging, perhaps inevitably, is supported by many studies in the early literature on the subject [1–3] . More recent publications have amply confirmed that declining mGFR occurs in ‘healthy’ and even ‘very-healthy’ subjects, such as living related donors for kidney transplantation [4, 5] . In addition, numerous epidemiological studies have described such declining GFR with aging in samples of apparently healthy populations studied with estimated GFR (eGFR) rather than mGFR [6, 7] . The studies with mGFR were limited to a relatively small sample size, especially in the elderly, whereas the epidemiological studies may have been confounded by inaccuracies in the eGFR equations in elderly subjects. Moreover, all these studies shared the same important limitation: they are cross-sectional in character and thus impaired by survival biases. Very few longitudinal studies are available in the literature [8] . The ‘Baltimore Longitudinal Study of Aging’, published in 1985, was seminal and frequently quoted [9] . Between 1958 and 1981, this study followed 254 apparently ‘healthy’ males, of varying age, with serial endogenous true creatinine clearances (Ccr), as an approximation of GFR. They found that the mean decline in Ccr was 0.75 ml/min/year, Received: July 10, 2015 Accepted: July 31, 2015 Published online: August 21, 2015

Glomerular Filtration Rate and Aging: Another Longitudinal Study - A Long Time Coming!