GLOMERULAR FILTRATION RATE

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

Contents

Measurement

Measurement using inulin

Estimation using creatinine clearance

Estimation using Cockcroft-Gault formula

MDRD formula

Calculation using Starling equation

Normal ranges

See also

References

External links

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).

Estimation using Cockcroft-Gault formula

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

MDRD formula

The MDRD formula estimates GFR using serum creatinine and age. Multipliers are used to adjust the estimation for race and gender.^[3]
$mbox\{eGFR\}\; =\; mbox\{186\}\; imes\; mbox\{Serum\; Creatinine\}^\{-1.154\}\; imes\; mbox\{Age\}^\{-0.203\}\; imes\; mbox\{1.21\; if\; Black\}\; imes\; mbox\{0.742\; if\; Female\}$

Calculation using Starling equation

It is also theoretically possible to calculate GFR using the Starling equation.^[4]
:$J\_v\; =\; K\_f\; (\; [P\_c\; -\; P\_i]\; -\; sigma[pi\_c\; -\; pi\_i]\; )$
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'
''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 $(\; [P\_c\; -\; P\_i]\; -\; sigma[pi\_c\; -\; pi\_i]\; )$ 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:^[5]

★ 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.

See also

★ Renal failure

★ Dialysis

★ Tubuloglomerular feedback

★ filtration fraction

References

1. - "Glomerular Filtration Rate"
2. GFR Calculator at cato.at - Cockcroft-Gault - GFR calculation (Cockcroft-Gault formula)
3. Evaluation of Laboratory Measures for Clinical Assessment of Kidney Disease -- Guideline 4. Estimation of GFR National Kidney Foundation
4. - "Forces Driving the Glomerular Filtration Rate":
5. Creatinine clearance at merck.com - The normal ranges of GFR.

External links

★ National Kidney Disease Education Program website. Includes professional references and GFR calculators

★ Online GFR Calculator using MDRD, Gault, and Combo Also shows GFR expected for age and current GFR percentage of expected.

★ Online GFR Calculator (Cockcroft-Gault Formula)

★ Online GFR Calculator - AnaemiaWorld.com

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