**10. Conclusions and perspectives**

In this chapter, we reviewed all the reference methods available in 2011 to measure GFR. Among these methods, inulin clearance can certainly be considered as the gold-standard because it is historically the first method used and because this marker is certainly the best characterized from a physiological point of view. However, inulin is expensive and commercial sources are limited (Gaspari et al., 1997). Due to its high molecular weight, there are doubts to use inulin in simplified plasma clearance (urinary clearances with constant infusion rate remain necessary but are very cumbersome). Measurement of plasma inulin is neither easy nor standardized. For all these reasons, the use of inulin is and will always be relatively marginal. In 2011, it is maybe time to move from the perfect physiological marker (inulin) to markers, maybe less perfect in the renal physiologic handling, but less costly, easier to use everywhere in the world and with a standardized measurement. From our point of view, iohexol is probably the best marker with the best balance between

How Measuring Glomerular Filtration Rate? Comparison of Reference Methods 47

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Estimation of Glomerular Filtration Rate by Creatinine-based Formulas: Current

physiological characteristics and practical advantages. Additional studies comparing references markers seem necessary in 2011. It seems also important to underline that GFR measurement is also subject to its own imprecision and to biological variation (Kwong et al., 2010). Therefore, it is illusionary to expect differences between different GFR methods of less than 10% (±2SD around the bias) and accuracy 10% over 85-90%. We must also keep these results in mind when we analyze the studies testing the performance of the creatinine-based equations (Kwong et al., 2010).
