How to Calculate GFR

By Allan Robinson

The rate at which fluid is filtered through the kidneys is known as the glomerular filtration rate (GFR). The GFR may be calculated directly by measuring the concentration of a substance that has a steady level in the blood and is freely filtered by the kidneys. There are also a number of methods that allow for the accurate estimation of the GFR from the measurement of various substances in the blood and urine.

Define the GFR formally. The GFR is the rate at which fluid is filtered from the renal capillaries in the Bowman's capsule. The GFR is therefore measured in volume per unit time, typically milliliters per minute (ml/min).

Identify the requirements of the test substance needed to calculate the GFR. The test substance must be freely filtered by the kidneys, meaning that it is neither absorbed nor secreted by the kidneys. Inulin is the only known substance that has this property.

Obtain the measurements needed to calculate the GFR. Inject inulin into the bloodstream. Measure the concentration of inulin in the blood, the concentration of inulin in the urine, the amount of urine excreted and the time required to excrete the urine.

Calculate the GFR directly. This can be calculated as (Uc x Uf)/Pc where Uc is the concentration of a test substance, Uf is the flow of urine and Pc is the concentration of the test material in the blood. The urine flow may be obtained by dividing the volume of urine by the time required to excrete it.

Identify normal GFR values. A GFR rate of at least 60 ml/min is generally adequate, although a significant decline from a previous reading may indicate early renal disease even if the GFR is still above this minimum value.

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