2 Clearance & GFR
What is clearance, and how can it be used to measure GFR?
The clearance of a substance is the volume of plasma that is cleared of the substance in a unit time. It has the units of volume over time, e.g. ml/min.
The clearance of a substance is calculated from the following equation:
[latex]C = \frac{UV}{P}[/latex]
Where:
| Symbol | Meaning | Example units |
| C | clearance | ml/min, μl/min |
| U | concentration / amount in the urine | mM, mg/ml |
| P | concentration in the plasma | mM, mg/ml |
| V | urine flow | ml/min, μl/min |
We can use the clearance equation to calculate GFR, however the substance used in the calculation has to meet a number of strict criteria.
- It has to be freely filtered.
- It must not be reabsorbed by the nephron.
- It must not be secreted by the nephron.
- It must not be metabolised in the nephron.
- It must not be synthesised in the nephron.
If these 5 criteria are met then the clearance of the substance = GFR. A substance that meets these criteria is inulin, which is not endogenously found in the body, but is often used in renal research.
Clinically, creatinine can be used. It doesn't completely fulfil the 5 criteria, but it's close enough. In the clinic, if you suspect a patient has renal issues associated with a drop in GFR (renal failure), the last thing you want to do is introduce an exogenous substance such as inulin. It's much better to use a substance that is endogenous.
Comparing clearance values across different substances
You can compare the clearance of any ion or solute to the clearance of inulin or creatinine. This comparison gives you information how those ions and solutes are handled by the kidney.
For example, if the clearance of your substance of interest is less than inulin, then this indicates that it is reabsorbed. On the other hand if the clearance of your substance of interest is greater than inulin, then this indicates that it is secreted.
Example calculation
For example. You measure the following parameters:
Urinary inulin concentration = 5 mM
Plasma inulin concentration = 42 mM
Urine flow rate = 15 ml/min
[latex]GFR = \frac{UV}{P}[/latex]
Before completing the calculation we need to review the units, to ensure they align.
[latex]GFR = \frac{mM \cdot \text{ml/min}}{mM}[/latex]
In this case the mM both cancel out, leaving units of ml/min for the GFR.
[latex]GFR = \frac{5 \cdot 15}{42}[/latex]
GFR = 126 ml/min
