Glomerular filtration rate

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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."[1] The GFR if used to measure renal function in patients with acute kidney injury or chronic kidney disease.[2]

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).[3] 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.[4][3] The equations have not been validated in acute renal failure.

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:[5]

MDRD original 6-variable formula

The two additional variables are the blood urea nitrogen and albumin levels:[6]

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:[6]

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).

The creatinine clearance systematically overestimates the GFR due to excretion creatinine by the renal tubules. The correction factor is below:[6]

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.[7] However, it does not correlate as strongly with the GFR as do the MRDR formula.[3]

The Estimated Creatinine Clearance then estimates GFR:[6]

Calculation using Starling equation

It is also theoretically possible to calculate GFR using the Starling equation.[8]

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 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[9]
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:[9]

Values are about 10% less for females.[8]






References

  1. Anonymous. Glomerular filtration rate. National Library of Medicine. Retrieved on 2008-01-08.
  2. Stevens LA, Coresh J, Greene T, Levey AS. Assessing kidney function--measured and estimated glomerular filtration rate. N Engl J Med. 2006 Jun 8;354(23):2473-83. PMID 16760447
  3. 3.0 3.1 3.2 Levey AS, Coresh J, Greene T, et al (2006). "Using standardized serum creatinine values in the modification of diet in renal disease study equation for estimating glomerular filtration rate". Ann. Intern. Med. 145 (4): 247–54. PMID 16908915[e]
  4. Rule AD, Larson TS, Bergstralh EJ, Slezak JM, Jacobsen SJ, Cosio FG (2004). "Using serum creatinine to estimate glomerular filtration rate: accuracy in good health and in chronic kidney disease". Ann. Intern. Med. 141 (12): 929–37. PMID 15611490[e]
  5. (2002) "K/DOQI clinical practice guidelines for chronic kidney disease: evaluation, classification, and stratification". Am. J. Kidney Dis. 39 (2 Suppl 1): S1–266. PMID 11904577[e]
  6. 6.0 6.1 6.2 6.3 Levey AS, Bosch JP, Lewis JB, Greene T, Rogers N, Roth D (1999). "A more accurate method to estimate glomerular filtration rate from serum creatinine: a new prediction equation. Modification of Diet in Renal Disease Study Group". Ann. Intern. Med. 130 (6): 461–70. PMID 10075613[e]
  7. GFR Calculator at cato.at - Cockcroft-Gault - GFR calculation (Cockcroft-Gault formula)
  8. 8.0 8.1 8.2 Ganong, William F. (2005). Review of medical physiology. McGraw-Hill Medical. ISBN 0-07-144040-2.  Cite error: Invalid <ref> tag; name "isbn0-07-144040-2" defined multiple times with different content Cite error: Invalid <ref> tag; name "isbn0-07-144040-2" defined multiple times with different content
  9. 9.0 9.1 Anonymous. GFR Frequently Asked Questions - NKDEP. National Kidney Disease Education Program. Retrieved on 2008-01-08.

See also


External links