Renal clearance, also called as Plasma clearance, is defined
as the amount of plasma that is cleared off a substance in a given unit of time
It is calculated by Ficks principle
Formula to calculate clearance value
Principle
- A known amount of a substance is injected into the body.
- After sometime, the concentration of the substance in plasma
and urine and the volume of urine excreted are estimated
Determination of clearance value for certain substances help
in assessing the renal functions, like:
- Glomerular filtration rate
- Renal plasma flow
- Renal blood flow
Measurement of Glomerular Filtration Rate (GFR)
- To measure GFR a substance should be used which is
completely filtered but neither reabsorbed nor secreted.
- Inulin is the ideal substance used to measure GFR as it is
completely filtered and neither reabsorbed nor secreted
- Hence inulin clearance indicates GFR
- Example:
Creatinine clearance is also used to measure GFR
- It is easier compared to inulin clearance as
- Creatinine is already present in body fluids and its plasma
concentration is steady throughout the day
- It is completely filtered and being a metabolite it is
neither reabsorbed nor secreted
Measurement of Renal Plasma Flow (RPF)
- To measure renal plasma flow, a substance should be used
which is filtered and secreted but not reabsorbed
- Paraaminohippuric acid is a substance which is used to
measure RPF as it is filtered and secreted but not reabsorbed
- Example:
Another substance Diodrast can also be used to
measure RPF
Measurement of Renal Blood Flow (RBF)
To measure RBF we need to know
- Renal plasma flow
- Percentage of plasma volume in blood
Renal plasma flow is measured using PAH clearance
Percentage of plasma volume in blood is determined by using
packed cell volume (PCV)
If PCV = 45%, Plasma volume in the blood = 100 – 45 = 55%,
that is 55 mL of plasma is present in every 100 mL of blood
Renal blood flow is calculated as
Urea Clearance Test
- It is a clinical test to assess renal function by using clearance
of urea from plasma by kidney every minute
- This test requires a blood sample to determine urea level in
blood and two urine sample collected at 1 hour interval to determine the urea
cleared by kidneys into urine
- Normal value of urea clearance is 70 mL/min
- Urea is a waste product formed during protein metabolism and
excreted in urine
- Hence, determination of urea clearance forms a specific test
to assess renal function
