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 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 of the GFR

In comparing the methods detailed below, the original 6 variable MDRD correlates slightly better with the GFR than the revised 4-variable formula (R²=0.890 versus R²=0.882). Both MDRD equations are better than Cockcroft-Gault formulae.

The MDRD 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.

When estimating the GFR "in very large or very small patients, multiply the reported eGFR by the estimated body-surface area (BSA) in order to obtain eGFR in units of mL/min":

${\mbox{eGFR}}/1.73m^{2}\times {\mbox{estimated BSA}}={\mbox{eGFR for drug dosing}}$

Modification of Diet in Renal Disease (MDRD) equations

MDRD 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:

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

MDRD original 6-variable formula

The two additional variables are the blood urea nitrogen and albumin levels:

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

Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation

The CKD-EPI may be more accurate than the MDRD equations, especially at higher GFRs

The CKD-EPI calculation uses age, race, gender, and serum creatinine (creatinine method must be traceable to isotope dilution mass spectrometry - IDMS) and is available online at http://mdrd.com/ and http://www.qxmd.com/calculate-online/nephrology/ckd-epi-egfr .

Estimation using creatinine clearance

Food and Drug Administration provides guidance on the labeling of prescription medications to guide dosing for patients with impaired renal function. Although this categorization uses estimated creatinine clearance, using estimated glomerular filtration yields similar recommendations for dosing adjustments.

The Estimated Creatinine Clearance is higher than the estimates GFR because of tubular secretion of creatinine:

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

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.

Reporting eGFRs

Reporting eGFRs may increase rates of referrals to nephrologists.