RENAL CLEARANCE

1. How can FF be determined from GFR and RPF? Knowing the PCV and RPF, how would one calculate the RBF?

2. Why is the creatinine concentration relatively constant?

3. Be familiar with why the determination of GFR by creatinine clearance is useful for the assessment and extent of kidney disease.

Renal clearance is a measurement of the kidney’s ability to remove substances from the plasma, and it has been useful for understanding basic elements of kidney function. Only a few substances are completely removed by the blood as it circulates through the kidneys so that a renal clearance measurement for a selected substance only provides a comparison with those that are. Accordingly, renal clearance measurements are made using selected substances for determining RPF and GFR. Values for GFR and RPF are used to determine FF, and the value determined for RPF, when coupled with the packed cell volume, can be used to calculate the RBF.

Creatinine Clearance Application

Creatinine is a nitrogenous byproduct of muscle metabolism. The major reaction that produces creatinine is the spontaneous loss of phosphoric acid from creatine phosphate in muscle (see Chapter 8, Muscle). Creatinine production is independent of protein metabolism. The amount produced depends on the mass of muscle in the body and is very constant from day to day. Because it is constantly produced it is constantly excreted. Normal creatinine concentrations are 0.5 to 2.0 mg/dL.

Creatinine clearance is a useful clinical assessment for the evaluation of kidney disease. In many animals (e.g., the dog) creatinine is freely filtered, it is not reabsorbed, nor is it secreted, the same as the selected substance used for the determination of GFR. Therefore, its measurement provides an estimate of GFR. The endogenous creatinine clearance method is most often used for this purpose. It is endogenous in that it uses the amount that is normally present in blood and does not require the infusion of creatinine to complement the amount normally present.

Creatinine clearance as a measurement of GFR can be used clinically for the assessment of kidney function because creatinine clearance measurement of GFR is directly related to the functional renal mass. Accordingly, a loss of nephron numbers by kidney disease can be confirmed by a corresponding decrease in GFR. Normal values for endogenous creatinine clearance in the dog are between.2 and 4 mL/min/kg body weight.

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