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Measuring renal function

Normal value for glomerular filtration rate (GFR) = 120 +/- 25 ml/min (males 125, females 115)

Urea blood test measured in a 'U+E' or urea and electrolytes) is a poor indicator of GFR as it varies with protein intake, liver function (it is generated in the liver) and state of hydration. More is reabsorbed from the renal tubules when urine is highly concentrated, and less during polyuria.

Causes of high Urea:

Creatinine is more reliable as it is produced by muscle at a constant rate ( dependent on muscle mass) and is almost entirely filtered at the glomerulus. However its serum levels may not rise out of the normal range until substantial renal impairment exists – normal values can conceal a reduction of up to 50% in GFR.

Creatinine clearance (CrCl; from a 24h urine collection) measurement can circumvent this problem, but 24h urine collections are inconvenient and of variable reliability. CrCl  tends to overestimate true GFR when renal function is poor. The Cockcroft-Gault formula estimates creatinine clearance from serum creatinine values (multiply by 0.85 for females because of relatively lower muscle mass):
 
(140-age) x weight x 1.23 x (0.85 if female)
Creat[micromol/l]

More info on Cockroft-Gault from the EdRen Handbook
 

eGFR  More accurate predictors of GFR than this exist now. The best tested is the MDRD equation to calculate estimated GFR (eGFR) using serum creatinine, age, sex. It is very widely used and reported so we have a separate page on it. More info from the edrep page on eGFR

Direct measurement of GFR Isotope tests or compounds such as inulin are used to measure GFR more directly. Markers (e.g. 51Cr-EDTA, 99Tc-DTPA) that are cleared almost entirely by glomerular filtration are infused, and their rate of disappearance from the circulation measured.


Reciprocal of creatinine plots

Plots of the reciprocal of creatinine show how the plasma creatinine concentration changes with time. Declining renal function often follows a linear progression on these charts, but in fact a graph of eGFR will show just the same info if you have those values too. These are useful for predicting an approximate date of ESRF, and to identify changes in the rate of progression.

Blank charts for plotting creatinine changes can be downloaded from the Edren handbook page on measuring GFR
 

Further info

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