The best indicator of glomerular function, and thus kidney function, is the glomerular filtration rate. Unfortunately assessment of the GFR is restricted to an academic exercise because we have no suitable endogenous marker to use in routine laboratory testing.
The ideal serum marker
For routine measurement of the GFR we require some kind of serum marker with the following properties:
- Constant rate of endogenous production
- Free glomerular filtration
- No tubular secretion or reabsorption
- No extrarenal elimination
Unfortunately no such endogenous substance exists (that we have identified) so we require exogenous markers to directly measure GFR. Excluding radioisotopes like technetium-99m, the two most promising exogenous markers are inulin and iohexol.
Estimating GFR
Because we have no readily available way to measure GFR clinically, various equations have been developed to estimate the GFR using the non-ideal endogenous markers, the most important of which is creatinine.
Creatinine clearance
Creatinine is produced by the body at a constant rate (depending on muscle mass) and is cleared principally by the kidney. The measurement of creatinine clearance is complicated by the usual unreliability of urine collection. And also by the fact that creatinine is actively secreted by the renal tubule; consequently, creatinine clearance is know to overestimate the GFR by 10-20%. Clearance is calculated by:
The eGFR
The author wont detail the equations that calculate eGFR because he doesn’t particularly care. Simply, the eGFR is usually just another way of reporting the serum creatinine so it isn’t really very useful.