It is a polymer of fructose which is freely filtered by the glomerulus, and is neither reabsorbed nor secreted by the renal tubules. It is metabolically inert and cleared only by the kidney. However, analysis of inulin is technically demanding and time-consuming. Conventional methods for estimating inulin clearance involve the measurement of inulin concentrations in both serum and timed urine collections. Ideally, the serum levels are measured at the mid-point of the timed urine collection.
Problems remain, however.
GFR estimation using both plasma and urine samples, as described above for inulin, are labor-intensive and costly. Moreover, obtaining accurate, timed urine collections, especially in infants, non-toilet trained children, and patients with urological abnormalities, poses additional challenges. To overcome these difficulties, methods using plasma clearance of exogenous markers for GFR estimation have been described. These markers may be given by single bolus, and GFR calculated from the rate of disappearance or, alternatively, steady state plasma clearance may be measured by infusing the filtration marker at a known rate until steady state is reached. At this point, the rate of elimination is equal to the rate of infusion.