Glomerular filtration rate

Glomerular filtration rate (GFR) is "the volume of water filtered out of plasma through glomerular capillary walls into Bowman's capsules per unit of time. It is considered to be equivalent to inulin clearance." The GFR if used to measure renal function in patients with acute kidney injury or chronic kidney disease.

Measurement

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

Measurement using inulin

The GFR is most accurately 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.

However, due to difficulties with accurately infusing inulin, various easier methods of estimating the GFR are available.

Estimation

Modification of Diet in Renal Disease revised 4-variable formula

The most commonly used formula is the "4-variable MDRD" which estimates GFR using four variables - serum creatinine, age, race, and gender:

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

Modification of Diet in Renal Disease original 6-variable formula

The original 6 variable MDRD correlates slightly better with the GFR than the revised 4-variable formula (R²=0.890 versus R²=0.882). The additional variables are the blood urea nitrogen and albumin levels:

${\mbox{eGFR}}={\mbox{170}}\ \times \ {\mbox{Serum Creatinine}}^{-0.999}\ \times \ {\mbox{Age}}^{-0.176}\ \times \ {\mbox{BUN}}^{-0.170}\times \ {\mbox{Albumin}}^{+0.3189}\ \ \times \ {\mbox{1.18 if Black}}\ \times \ {\mbox{0.762 if Female}}$

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.

Estimation using creatinine clearance

Using a direct measurement of the creatinine clearance

By measuring the amount of creatinine excreted in the urine over one day, the creatinine clearance may be calculated. 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 formula for the creatinine clearance is:

${\mbox{Creatinine clearance }}={\frac {\frac {\mbox{ Amount of creatinine in urine (mg)}}{\mbox{Duration of the urine collection (minutes)}}}{{\mbox{Plasma creatinine concentration (mg}}/{\mbox{ml)}}}}$

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).

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

The creatinine clearance systematically overestimates the GFR due to excretion creatinine by the renal tubules. The correction factor is below:

${\mbox{Glomerlular filtration rate}}={\mbox{0.81}}\ \times \ {\mbox{Creatinine clearance}}$

Using an Cockcroft-Gault estimation the creatinine clearance

The Cockcroft-Gault formula may be used to estimate the creatinine clearance without having to collect urine over a period of time. However, it does not correlate as strongly with the GFR as do the MRDR formula.

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

The Estimated Creatinine Clearance then estimates GFR:

${\mbox{Glomerlular filtration rate}}={\mbox{0.84}}\ \times \ {\mbox{Cockcroft-Gault formula}}$

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:

General usage	Glomerular usage	Meaning of variable	Relationship to GFR	Description
Pc	Pgc	Capillary hydrostatic pressure	Direct	Increased by dilation of afferent arteriole or constriction of efferent arteriole
Pi	Pbs	Interstitial hydrostatic pressure	Inverse
πc	πgc	Capillary oncotic pressure	Inverse	Decreased by nephrotic syndrome
πi	πbs	Interstitial oncotic pressure	Direct
Kf	Kf	Filtration coefficient	Direct	Increased by inflammation
σ	σ	Reflection coefficient	Inverse
Jv	GFR	net filtration	n/a

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.

For example, 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.

Normal ranges

Normal values for eGFRs

Age (Years)	Mean eGFR
20-29	116 mL/min/1.73 m2
30-39	107 mL/min/1.73 m2
40-49	99 mL/min/1.73 m2
50-59	93 mL/min/1.73 m2
60-69	85 mL/min/1.73 m2
70+	75 mL/min/1.73 m2

The normal ranges of GFR, adjusted for body surface area, are:

Values are about 10% less for females.

See also

• Acute kidney injury
• Chronic kidney disease

External links

• National Kidney Disease Education Program website. Includes GFR calculators