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Measuring GFR and VUR

Measuring GFR and VUR

The kidneys are highly vascular organs that filter the blood, removing wastes for excretion and returning the rest of the blood to the vascular system. The functional units of the kidneys are nephrons, each composed of a glomerulus and a tubule.

How is the GFR measured? What are the strengths and limitations of methods of determining GFR?

Often an incidental finding in children during a urinary tract infection workup, in vesicoureteral reflux (VUR), urine flows from the bladder back up the ureters.

Could you explain the mechanism of action of this abnormality in children?

Measuring GFR and VUR

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APA

Measuring GFR and VUR

Glomerular Filtration Rate (GFR) Measurement

The Glomerular Filtration Rate (GFR) is a key indicator of kidney function, reflecting the rate at which the kidneys filter blood. It is used to assess kidney health and diagnose potential kidney diseases. GFR is typically measured using the creatinine clearance test or can be estimated using formulas like the Cockcroft-Gault or MDRD equations.

Methods of Measuring GFR:

  1. Creatinine Clearance Test:

    • Procedure: This involves measuring the amount of creatinine in the urine over a 24-hour period and comparing it to the creatinine level in the blood. Creatinine, a waste product of muscle metabolism, is freely filtered by the kidneys and is not reabsorbed.
    • Calculation: The formula used is: Creatinine Clearance (mL/min)=Urine Creatinine (mg/dL)×Urine Volume (mL)Plasma Creatinine (mg/dL)×1440\text{Creatinine Clearance (mL/min)} = \frac{\text{Urine Creatinine (mg/dL)} \times \text{Urine Volume (mL)}}{\text{Plasma Creatinine (mg/dL)} \times 1440}
    • Strengths: It is a direct measure of kidney function and can be used to calculate the GFR accurately, especially in people with normal kidney function.
    • Limitations: It requires a 24-hour urine collection, which is inconvenient and may lead to inaccuracies in the collection process (e.g., missing urine or incomplete collection). Additionally, creatinine secretion by muscles may cause variability in results, particularly in patients with fluctuating muscle mass.

  2. Estimation Equations:

    • Cockcroft-Gault Equation: GFR=(140−age)×weight (kg)72×serum creatinine (mg/dL)\text{GFR} = \frac{(140 – \text{age}) \times \text{weight (kg)}}{72 \times \text{serum creatinine (mg/dL)}} For females, multiply by 0.85.
    • MDRD Equation (Modification of Diet in Renal Disease): GFR=175×serum creatinine−1.154×age−0.203×(0.742 for females)×(1.210 for Black individuals)\text{GFR} = 175 \times \text{serum creatinine}^{-1.154} \times \text{age}^{-0.203} \times (0.742 \text{ for females}) \times (1.210 \text{ for Black individuals})
    • Strengths: These methods are non-invasive and require only a blood sample. They are widely used in clinical practice, especially for estimating kidney function in routine care.
    • Limitations: These equations may be less accurate in extreme body sizes (e.g., very obese or malnourished patients), in children, or in those with abnormal muscle mass. The equations are based on creatinine levels, which may not always accurately reflect kidney function in certain individuals (e.g., in elderly patients, pregnant women, or those with…