Updated: 2/13/2019

Renal Clearance

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  • Introduction
    • Renal clearance
      • describes the amount (volume) of plasma that is completely cleared by the kidneys per unit time
        • in other words it reflects the excretion of a substance (e.g., medications)
        • this depends on
          • glomerular filtration
          • tubular reabsorption and secretion
        • clearance decreases gradually with age but may have acute changes due to pathology
      • equation
        • C(x) = (UxV) / Px
          • C(x) = clearance of substance X (mL/min)
          • Ux = urine concentration of substance X (mg/mL)
          • V = urine flow rate (mL/min)
          • Px = plasma concentration of substance X (mg/mL)
    • Clearance ratios
      • C(inulin)
        • C(inulin) = GFR
        • inulin is freely filtered across glomerular capillaries
          • it is neither reabsorbed or secreted
            • amount of inulin filtered equals amount of inulin excreted
        • inulin is a reference substance called a glomerular marker
      • C(x) / C(inulin) > 1.0
        • C(x) > GFR
        • interpretation
          • there is a net secretion of substance X
            • e.g., para-aminohippuric acid
              • can be used to determine renal plasma flow
      • C(x) / C(inulin) < 1.0
        • C(x) < GFR
        • interpretation
          • there is a net reabsorption of subtance X
            • e.g., glucose
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(M1.RL.17.4695) A 45-year-old gentleman with a history of poorly controlled diabetes mellitus is referred to a nephrologist for evaluation of the possibility of early stage kidney failure. Upon evaluation, the nephrologist decides to assess the patient's renal plasma flow by performing a laboratory test. Which of the following substances would be the best for estimating this value?

QID: 107948

Creatinine

21%

(80/388)

Inulin

21%

(83/388)

Glucose

1%

(3/388)

Para-aminohippurate (PAH)

53%

(207/388)

Urea

2%

(9/388)

M 1 C

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