Renal clearance refers to the volume of plasma that is cleared of a particular substance in a given time, ussually 1 minute. Renal clearance tests are done to determine the glomerulus filtration rate, which allows us to detect glomerular damage and follow the progress of renal disease.
The renal clearance rate (RC) of any substance, in ml/min, is calculated from the equation
RC=Renal clearance rate
U=concentration of the substance in urine (mg/ml)
V=flow rate of urine formation (ml/min)
P=concentration of the substance in plasma (ml/min)
Because it is freely filtered and neither reabsorbed nor secreted by the kidneys, inulin is the standard used to determined the glomerulus filtration rate. a polysaccharide with a molecular weight of approximately 5000, inulin has a renal clearance value equal to the glomerulus filtration rate. When inulin is infused such that its plasma concentration is 1 mg/ml (P=1mg/ml), then generally U=125mg/ml, and V=1ml/min. Therefore, its renal clearance is RC=(125×1)/1=125ml/min, meaning that in 1 minute the kidneys have removed (cleared) all the inulin present in 125 ml of plasma.
The clearance value tell us about the net handling of a substance by the kidneys.
There are three possible cases :
- A clearance values less than that inulin means that a substance is reabsorbed. An example is urea with an RC of 70 ml/min, meaning that of the 125 ml of glomerular filtrate formed each minute, approximately 70 ml is completely cleared of urea, while the urea in the remaining 55 ml is recovered and returned to the plasma. If the RC is zero (such as for glucose in healthy individuals), reabsorption is complete or the substance is not filtered.
- If the RC is equal to that of inulin, there is no reabsorption or secretion.
- If the RC is greater than that of inulin, the tubule cells are secreting the substance into the filtrate. This is the case with the most drug metabolites. Knowing a drug’s renal clearance value is essential because of its high, the drug dosage must also be high and administered frequently to maintain a therapeutic level.
Creatinin, which has an RC of 140 ml/min. is freely filtered but also secreted in small amounts. It is often used nevertheles to give a “quick and dirty” estimate of glomerulus filtration rate.