Measuring glomerular filtration rate of the kidneys with inulin

In chronic kidney disease, the kidneys lose their ability to effectively filter waste products in the blood because of damage to the glomeruli of nephrons. Kidney function is determined by measuring glomerular filtration rate (GFR)--the volume of plasma that the kidneys filter through the glomeruli per unit time. The “gold standard” for measuring GFR is through the use of inulin, a carbohydrate produced by many plants, such as onions and garlic. Since inulin is not endogenous in humans, a specified mass must be injected into a person’s bloodstream in order to measure GFR.

Inulin is useful as an indicator of GFR because the kidneys handle it in a unique way. Unlike most other substances in the blood, inulin is neither reabsorbed into the blood after filtration nor secreted through peritubular capillaries. Thus, the amount of inulin cleared through the urine is indicative of the amount of plasma filtered by the body’s glomeruli. GFR can be calculated using Equation 1.

Equation 1 - Urine Concentration refers to the concentration of inulin in a sample of urine, Urine Flow refers to the amount of urine produced in a given time period, and Plasma Concentration refers to the concentration of inulin in blood plasma after intravenous injection.

Researchers investigated how GFR was affected by furosemide, a diuretic medication that prevents sodium reabsorption in the loop of Henle. To do so, they observed the clearance rate of inulin in three groups of healthy cats that received different treatments. Their results are shown in Table 1.

Table 1: Group A (control) received no treatment. Group B received 20 mL/h intravenous saline solution. Group C received 20 mL/h intravenous saline solution with 1 mg/h furosemide.