Categories: Nephrology

Why does Creatinine Clearance overestimate GFR?

Creatinine is a metabolic byproduct generated from muscle breakdown. Creatinine is filtered from blood by the kidneys and is not reabsorbed in any appreciable amount, hence is a proxy of how well the kidneys filter blood.

Creatinine clearance (CrCl) is used to estimate Glomerular filtration rate (GFR) in mL/min. It is measured via a 24-hour urine collection.

Why does it overestimate? Due to tubular secretion of Creatinine. There will be filtered Cr + tubular secreted Cr in the urine collection. Making the GFR appear higher than it actually is when calculated.

NOTE: Certain drugs (i.e trimethoprim, cimetidine) reduce tubular secretion of Creatinine and actually provide a more accurate measure of GFR

RULE OF THUMB: Serum Cr will vary with the amount of muscle tissue. Hence in older patients and women, CrCl can appear normal while GFR is markedly reduced.

REFERENCES

  1. Rosner MH, Bolton WK. Renal function testing. Am J Kidney Dis. Jan 2006;47(1):174-83.
  2. Nankivell, BJ. Creatinine clearance and the assessment of renal function. Aust Prescr 2001; 24:15-7.
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Dr. C Humphreys

Internal Medicine

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