Glomerular filtration rate

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Glomerular filtration rate (GFR) is the volume of fluid filtered from the renal glomerular capillaries into the Bowman's capsule per unit time.^[1] Clinically, this is often measured to determine renal function.

1 Measurement
2 Normal ranges
3 See also
4 References
5 External links

[edit] Measurement

There are several different techniques used to calculate or estimate the glomerular filtration rate.

[edit] Measurement using inulin

The GFR can be determined by injecting inulin (not insulin) into the plasma. Since inulin is not reabsorbed 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.

[edit] 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. The GFR is typically recorded in 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).

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

[edit] Estimation using Cockcroft-Gault formula

The Cockcroft-Gault formula may be used to calculate an Estimated Creatinine Clearance, which in turn estimates GFR:^[2]

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

[edit] Calculation using Starling equation

It is also theoretically possible to calculate GFR using the Starling equation.^[3]

J v = K f ([P c - P i] - σ[π c - π i])

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

General usage	Glomerular usage	Meaning of variable	Relationship to GFR	Description
P_c	P_gc	Capillary hydrostatic pressure	Direct	Increased by dilation of afferent arteriole or constriction of efferent arteriole
P_i	P_bs	Interstitial hydrostatic pressure	Inverse
π_c	π_gc	Capillary oncotic pressure	Inverse	Decreased by nephrotic syndrome
π_i	π_bs	Interstitial oncotic pressure	Direct
K_f	K_f	Filtration coefficient	Direct	Increased by inflammation
σ	σ	Reflection coefficient	Inverse
J_v	GFR	net filtration	n/a

Note that $([P c - P i] - σ[π c - π i])$ 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.