Renal Clearance

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:

1. Glomerular filtration rate
2. Renal plasma flow
3. 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

1. Renal plasma flow
2. 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