Clearance & eGFR
Clearance
Clearance is the virtual volume of plasma cleared from a substance per unit of time.
C = V*U/P
- C = clearance of the substance
- V = urinary flow rate
- U = urinary concentration of the substance
- P = plasma concentration of the substance
Glomerular filtration rate (GFR) and effective glomerular filtration rate (eGFR)
GFR is the volume of plasma fluid that enters into the Bowman’s capsule, per unit of time.
- It is determined by a sum of the hydrostatic and oncotic pressure gradients between the glomerular tuft (within the capillary) and the Bowman’s capsule.
- It is an excellent measurement of kidney function.
- Renal diseases usually decrease GFR.
- There are many factors that can influence GFR such as gender, age, kidney size, pregnancy, etc.
- An average value for GFR is 120-130ml/min/1.73m^2 depending on the gender.
- It’s impossible to measure GFR values directly, therefore we must rely on estimated values gathered through clearance or calculations.
We can estimate the GFR by measuring the clearance of substances that have little to no absorption or secretion in the urinary system (therefore, the clearance of the substance should be similar to the GFR). There are both endogenous and exogenous substances that may fit into this criteria, two out of which are