Renal: Measuring GFR with inulin

Kidney function is determined by measuring the glomerular filtration rate (GFR) – the volume of plasma that the kidneys filter through the glomeruli per unit time. The “gold standard” method for measuring GFR involves the use of inulin, a carbohydrate produced by plants.

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 Glomerular filtration rate calculation

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 the “loop diuretic” furosemide. Loop diuretics, including furosemide and bumetanide, bind to the co-transporter NKCC2 located in the luminal membrane of the thick ascending limb of the loop of Henle. By inhibiting NKCC2, loop diuretics disrupt this ion transport and reduce the reabsorption of cotransported ions. Researchers observed the clearance rate of inulin in three groups of healthy cats. The results are shown in Table 1.

Table 1 Measurements taken in control and experimental groups