**8. Conclusions and perspective**

GFR indexing for BSA has little influence on normal body sized people but can be misleading in people with unusual anthropometric data. GFR indexation is necessary to compare the GFR of patients with each other and with reference values. We showed that correcting for squared height or ECV are two good alternatives to normalize the GFR in children. It seems intuitively better to correct GFR for ECV than for BSA because the role of the kidneys is to regulate body fluid composition. Additional research concerning this topic is of particular interest. Also, the rate constant β or the mean transit time T is worth considering as an indicator for kidney function. It must be emphasized that it is important to examine whether other kidney function indicators lead to different clinical decisions.

#### **9. References**


as an excellent alternative for the radio-active markers. In 2010, Schwartz et al. showed how the GFR in children could be determined from the slow component of an iohexol clearance curve according to the equation GFR = 1.0019 \* slowGFR – 0.001258 \* slowGFR². (Schwartz et al., 2010) Since the literature often cautions that the equations are only valid in the populations similar to those in which they were developed, Derek et al. recently tried to develop, based on an iohexol study, an universal formula for use in adults as well as in children. (Derek et al., 2011) The equation of Derek et al. is already expressed in

The rate constant β, or even T, may be a perfect indicator for kidney function, which immediately allows comparison of the kidney function of different persons with each other. Simulations showed us that the cut-off value of 60 ml/min/1.73m² agreed with a β value of 0.004 min-1 or a transit time of 4 hours, a cut-off value of 30 ml/min/1.73m² agreed with a β

GFR indexing for BSA has little influence on normal body sized people but can be misleading in people with unusual anthropometric data. GFR indexation is necessary to compare the GFR of patients with each other and with reference values. We showed that correcting for squared height or ECV are two good alternatives to normalize the GFR in children. It seems intuitively better to correct GFR for ECV than for BSA because the role of the kidneys is to regulate body fluid composition. Additional research concerning this topic is of particular interest. Also, the rate constant β or the mean transit time T is worth considering as an indicator for kidney function. It must be emphasized that it is important to

examine whether other kidney function indicators lead to different clinical decisions.

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**8. Conclusions and perspective**

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**2** 

Pierre Delanaye

*Belgium* 

**How Measuring Glomerular Filtration Rate?** 

Glomerular filtration rate (GFR) is considered as the best way to assess global renal function (Gaspari et al., 1997; Stevens & Levey, 2009). Even if GFR estimations (based on creatinine- or cystatin C-based equations) are most often used (see Table 1)(Cockcroft & Gault, 1976; Levey et al., 1999; Levey et al., 2006; Levey et al., 2009), measuring "true" GFR is still important in clinical practice, especially in particular patients (Delanaye et al., 2011a; Delanaye & Cohen, 2008; Stevens & Levey, 2009). In this chapter, we will review the different markers which can be considered as reference methods to measure GFR. Before moving to clinical trials, we have to recall the physiological characteristics of an

The history of the renal physiology is deeply influenced by the book published by Homer W. Smith in 1951 (Figure 1) : « The kidney: structure and function in health and disease »(Smith, 1951b). In this best-seller of nephrology, Smith compiled all the physiological data (more than 2300 references) which have been published in the scientific literature until 1951. Smith, himself, has largely contributed to the physiological knowledge of the kidney. A large part of this book is dedicated to the GFR measurement. The concept of clearance is well explicated. Actually, the Danish physiologist, Poul Brandt Rheberg was the first to use and define the concept of clearance in 1926 even if this author did not use the word "clearance". Rheberg studied on himself the urea and creatinine clearances to prove that kidney has a filtrating and not only a secreting action (Rehberg, 1926b; Rehberg, 1926a). The term clearance was used for the first time by Möller in 1929 and was then concerning the urea clearance which was proposed as the first evaluation of renal function (Möller et al., 1929). Smith has largely contributed to make popular and classical this concept of clearance to assess GFR (Smith, 1951a). Renal clearance of a substance is defined as the volume of plasma cleared from this substance per time unit (mL/min). Clearance is thus a virtual volume but will permit to apprehend GFR and renal function. However, the concept of clearance is applicable to any internal or external substances. To be considered as a reference

**2. Clearance concept and ideal marker for glomerular filtration rate** 

method, a marker must have strict physiological characteristics (Smith, 1951b):

1. Marker production and marker plasma concentration must be constant if GFR does not

**1. Introduction** 

ideal GFR marker.

change

**Comparison of Reference Methods** 

*University of Liège, CHU Sart Tilman, Liège* 

children with chronic kidney disease. *Kidney International*, Vol.77, No.1, (January 2010) pp.65-71, ISSN 0085-2538

