Glomerular filtration rate

Overview
Glomerular filtration rate (GFR) is the volume of fluid filtered from the renal (kidney) glomerular capillaries into the Bowman's capsule per unit time. Clinically, this is often measured to determine renal function. Compare to filtration fraction.

Measurement
There are several different techniques used to calculate or estimate the glomerular filtration rate (GFR or eGFR).

Measurement using inulin
The GFR can be determined by injecting inulin (not insulin) into the plasma. Since inulin is neither reabsorbed nor secreted by the kidney after glomerular filtration, its rate of excretion is directly proportional to the rate of filtration of water and solutes across the glomerular filter.

Estimation using creatinine clearance
In clinical practice, however, creatinine clearance is used to measure GFR. Creatinine is an endogenous molecule, synthesized in the body, which is freely filtered by the glomerulus (but also secreted by the renal tubules in very small amounts). Creatinine clearance is therefore a close approximation of the GFR. However, the approximation of the GFR calculation is best measured by the evaluation and visualization of the frequency and duration of urination. The GFR is typically recorded in units of volume per time, e.g. milliliters per minute (ml/min).

Example: A person has a plasma creatinine concentration of 0.01 mg/ml and in 1 hour he excretes 75 mg of creatinine in the urine. The GFR is calculated as M/P (where M is the mass of creatinine excreted per unit time and P is the plasma concentration of creatinine).

$$\mbox{GFR }= \frac{\frac{75\mbox{ mg}}{60\mbox{ mins}}}{0.01\mbox{ mg}/\mbox{ml}} = 125 \mbox{ ml}/\mbox{min}$$

Estimation using Cockcroft-Gault formula
The Cockcroft-Gault formula may be used to calculate an Estimated Creatinine Clearance, which in turn estimates GFR:

$$\mbox{Creatinine clearance} = \frac { \mbox{(140 - Age)} \times \mbox{Mass (in kilograms)}} {\mbox{72} \times \mbox{Plasma Creatinine (in mg/dl)}} \times \mbox{0.85 if female}$$

Modification of Diet in Renal Disease (MDRD) formula
The most commonly used formula is the "4-variable MDRD" which estimates GFR using four variables: serum creatinine, age, race, and gender. The original MDRD used six variables with the additional variables being the blood urea nitrogen and albumin levels. The equations have been validated in patients with chronic kidney disease; however both versions underestimate the GFR in healthy patients with GFRs over 60 mL/min. The equations have not been validated in acute renal failure.

$$\mbox{eGFR} = \mbox{186}\ \times \ \mbox{Serum Creatinine}^{-1.154} \ \times \ \mbox{Age}^{-0.203} \ \times \ \mbox{1.21 if Black} \ \times \ \mbox{0.742 if Female}$$

Calculation using Starling equation
It is also theoretically possible to calculate GFR using the Starling equation.


 * $$J_v = K_f ( [P_c - P_i] - \sigma[\pi_c - \pi_i] )$$

The equation is used both in a general sense for all capillary flow, and in a specific sense for the glomerulus:

Note that $$ ( [P_c - P_i] - \sigma[\pi_c - \pi_i] )$$ is the net driving force, and therefore the net filtration is proportional to the net driving force.

In practice, it is not possible to identify the needed values for this equation, but the equation is still useful for understanding the factors which affect GFR, and providing a theoretical underpinning for the above calculations.

Normal ranges
The normal ranges of GFR, adjusted for body surface area, are:
 * Males: 70 ± 14 mL/min/m2
 * Females: 60 ± 10 mL/min/m2

GFR can increase due to hypoproteinemia because of the reduction in plasma oncotic pressure. GFR can also increase due to constriction of the efferent arteriole but decreases due to constriction of the afferent arteriole.

Related Chapters

 * Creatinine clearance'''
 * Renal failure
 * Dialysis
 * Tubuloglomerular feedback
 * filtration fraction