Handbooks > Unit handbook > GFR estimation
Glomerular filtration rate (GFR) is the usual method for measuring renal excretory function. Most commonly it is esti§mated (eGFR), or surrogates are used such as creatinine clearance (CrCl).
Normal values for GFR
120±25 mls/min (95th centiles. Males approx 5mls higher, females approx 5mls lower). Therefore values >90 are normal for all. Converting for average surface area (per 1.73m2) removes the sex difference.
With age, GFR tends to fall (to approx 100mls/min/1.73m2 at age 70), although serum creatinine does not rise much in healthy individuals (Fliser et al, Kidney Int 51:1196-1204, 1997). This fall is mostly due to subclinical pathology.
Estimating GFR
A variety of methods have been used. All are based on serum creatinine estimations. Although the Cockcroft-Gault formula is widely used, it was developed to permit estimation of CrCl, not of GFR. The best validated method comes from the MDRD Study data (Ann Int Med 130:461-70, 877-84, 1999). The study derived a number of equations. The abbreviated, or four-variable equation includes age, sex, creatinine, and race (black or not black). Adding more variables (albumin, urea) adds little to accuracy.
It is important to be aware of the limitations of these equations (more info on eGFR from the UK CKD eGuide):
- Accuracy - even for the MDRD equation, which is the best there is, the confidence limits are wide. 90% of values are within 30% of the true value; 98% within 50%.
- Extremes - none of the methods for estimating GFR without actual measurements of it are likely to be accurate in extreme examples of low muscle mass, or other unusual circumstances.
- Stability - for all methods, [creat] must not be changing quickly.
- Systematic errors - MDRD is better at low GFRs.
- Age - the Schartz or Counahan Barrat equation (requires height) should be used in children.
- Race, different equations - more info
Most UK labs now report eGFR when returning creatinine values. When available, these values should be more accurate than cacluations performed by you, as they should incorporate lab-specific correction factors. If not, here are some resources:
- The UK Renal Association Website – http://www.renal.org/eGFR
- Online calculator from nephron.com offers a simple abbreviated MDRD, and has a useful menu of alternatives including quaint US units and also Cockroft-Gault. Take care to set units for [creat] correctly.
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Prediction of eGFR Manual calculation is difficult so here are some resources:
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Measuring GFR
For direct measurement of GFR, isotope tests have largely taken the place of inulin clearance. Like inulin, these markers (eg 51Cr-EDTA, 99Tc-DTPA) are cleared almost entirely by glomerular filtration, and measures of their disappearance rate from the circulation, or appearance in the urine, can be used to estimate GFR.
GFR and Reciprocal of creatinine plots
Plots of GFR: time, or of the reciprocal of creatinine are useful as they can be used to give an approximate prediction of the date of ESRF for many patients, and to identify changes in the rate of progression. The rate of creatinine production in an individual changes slowly if at all. Note that linear plots of GFR/time are identical to reciprocal creatinine plots.
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Download a blank reciprocal creatinine plot (pdf file) but note that plots of GFR vs time should be linear and in the future will probably replace 1/creatinine. If you do not have a pdf file reader, one can be downloaded by clicking here |
An example of a reciprocal creatinine plot for a patient with an original diagnosis of small vessel vasculitis:
Adapted from Davidson's Textbook of Medicine, 18th edition 1999; Haslett, Chilvers et al eds. (Churchill Livingstone)
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Creatinine clearance
Creatinine clearance overestimates GFR in a variety of circumstances, because of tubular secretion. This is particularly important at low levels of GFR. To calculate it, pay close attention to units and remember that there are 1440 minutes in 24h (details of calculations below). However 24 hour urine collections are error-prone and have been shown to be less accurate for estimating GFR than deductions from plasma creatinine.
Calculations: everyone knows it's UV/P but the units get a little confusing. This formula shows SI units.
| UV | Amount of creatinine in 24h of urine is reported by the lab in mmol/24h. x1000 to convert to micromols |
| P | Serum creatinine is reported in micromols/l. x1000 to do calculation for mls |
| 1440 | Number of minutes in 24h |
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The Cockcroft-Gault equation aims to predict creatinine clearance from knowledge of serum creatinine, age and weight. See Calculator 2 (mentioned above) or do it the hard way:
(140-age) x weight x 1.23 x (0.85 if female)
Creat[micromol/l]
Weight should be lean body mass, but attempting to calculate that makes the equation unreasonably complicated. Estimate lean body mass for extremes of size, or use the MDRD equation instead, or best of all, measure it.
Note that these estimates/measures of CrCl are not normalised for body size, if you want to compare with eGFR you should adjust according to calculated surface area.
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