Clearance: Concept, Applications, and Considerations

Clearance: Concept, Applications, and Considerations

Clearance is a crucial index in evaluating kidney function. It reflects the glomerular filtration capacity for substances in the blood.

1. Concept:

• If a substance is completely filtered through the glomerulus, without further reabsorption or secretion in the renal tubules, its clearance equals the glomerular filtration rate (GFR).

• Clearance formula:

• Cx = Ux . V / Px

• Where:

• Ux (mg/mL): Concentration of substance X in urine

• V (mL/minute): Urine flow rate

• Px (mg/mL): Concentration of substance X in plasma

• Essentially, clearance is the volume of that substance in plasma filtered through the glomerular system.

• The product Ux . V is called the excretion rate (ER). According to the formula, clearance equals ER divided by the substance’s plasma concentration.

2. Glomerular Markers:

• Inulin is considered the “gold standard” for glomerular markers. However, Inulin is not widely used due to the complexity of measurement.

3. Creatinine Quantification for Clearance Measurement:

• The Jaffé colorimetric method in alkaline medium is used to quantify creatinine concentration in blood and urine.

• This method has two main errors:

• Creatinine is further secreted by the renal tubules, increasing creatinine concentration in urine by about 20%.

• The Jaffé technique can quantify other chromogens in plasma, falsely increasing creatinine concentration in plasma by about 20%.

• Due to these two errors canceling each other out, creatinine clearance can reasonably estimate GFR.

4. Creatinine Clearance and GFR:

• In chronic kidney disease, when GFR drops to 20 mL/minute, creatinine clearance can be artificially elevated up to 50%.

• Plasma creatinine concentration and GFR are inversely correlated in a “hyperpole” manner.

• When GFR is halved, plasma creatinine concentration doubles.

• At low GFR levels, a slight change in GFR leads to significant fluctuations in plasma creatinine concentration.

5. Plasma Creatinine Quantification Methods:

• Enzyme method: Not affected by protein levels in the body.

• Jaffé method: More common but affected by protein levels in the body.

6. GFR Estimation Using Formulas:

• Cockcroft Gault formula:

• C = ((140 – Age) x Weight) / (72 x [Creat ht])

• Notes:

• [Creat ht] is in mg/dL.

• This formula has relatively high sensitivity and accuracy in Vietnamese individuals.

• It needs to be adjusted according to body surface area.

• MDRD formula:

• Advantage: No need to adjust for body surface area.

• Disadvantage: Only applicable to individuals aged 18-70 with declining kidney function.

• Bedside Schwartz equation:

• Used for children.

• eGFR = (0.41 x height) / [Creat ht] adjusted for body surface area.

• eGFR estimation from Cystatin C: Can be a consideration.

7. Causes of Chronic Kidney Disease:

• Diabetes (40%)

• Hypertension (30%)

• In Vietnam, Hypertension is more prevalent than diabetes.

Conclusion:

Clearance is a critical index for assessing kidney function. Choosing appropriate measurement methods and GFR estimation formulas for each individual is crucial for timely treatment interventions.