Measuring renal function

Measuring renal function

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

Urea is the original blood test for kidney function, measured in a ‘U+E’ or urea and electrolytes. Urea 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. However these weaknesses as a test for kidney function can be useful when its result is compared with Creatinine.

Causes of high Urea:

  • Dehydration
  • Renal failure
  • GI bleeding (=large protein meal)

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)
More info on Cockroft-Gault from the EdRen Handbook. It isn’t really as useful as eGFR in the modern era, but prescribing instructions for some drugs use it.

eGFR  Estimated GFR; various equations exist to do this. Oldest and most widely used, but not necessarily the best, is the MDRD equation. We have a separate 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, or iohexol, or inulin) 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