eGFR (Estimated Glomerular Filtration Rate) - Kidney Function Tests - Inulin & Creatinine - Lab

Summary

In this video, the speaker delves into the concept of Glomerular Filtration Rate (GFR) as a critical measure of kidney function. They explain how GFR is estimated using substances like inulin, creatinine, and cystatin C, emphasizing the importance of plasma clearance. The speaker highlights the factors that influence GFR, including age and muscle mass, and discusses how GFR declines over time. Through clear examples, viewers are guided on calculating GFR and the significance of these measurements in assessing kidney health. The content is insightful for anyone looking to understand kidney physiology and lab testing methods.

Takeaways

• 😀 GFR (glomerular filtration rate) is a key test to assess kidney function, indicating how well the kidneys filter blood. A normal GFR is around 125 mL per minute.
• 😀 The kidneys filter plasma, not red blood cells, and this process is crucial for removing waste while retaining essential substances like sodium and glucose.
• 😀 The kidney's filtration process involves four key actions: filtration (blood to kidney), reabsorption (kidney to blood), secretion (blood to urine), and excretion (waste into urine).
• 😀 The volume of plasma filtered by the kidneys each minute is called the glomerular filtration rate (GFR), and it is typically 125 mL per minute.
• 😀 Cardiac output influences renal blood flow, with the kidneys receiving about 20-25% of the heart's output. This flow is then filtered to calculate the GFR.
• 😀 Plasma clearance is the volume of plasma cleared of waste products by the kidneys, and it's crucial for calculating GFR. This can be measured using substances like inulin or creatinine.
• 😀 Inulin is the ideal substance for measuring GFR because it is freely filtered and neither reabsorbed nor secreted by the kidneys.
• 😀 Creatinine is a common alternative for measuring GFR but can slightly overestimate it due to some secretion by the kidneys.
• 😀 Age, muscle mass, and gender impact GFR. With age, GFR typically declines by about 6.5 mL per minute per decade.
• 😀 Plasma clearance equations can be used to calculate GFR, where the volume of plasma cleared is divided by the concentration of the substance (e.g., inulin) in the plasma.
• 😀 For clinical purposes, creatinine and cystatin C are commonly used to estimate GFR, although inulin remains the most accurate test for kidney function.

Q & A

What is the estimated glomerular filtration rate (eGFR) and why is it important?

The estimated glomerular filtration rate (eGFR) is a measure of kidney function that estimates how well the kidneys are filtering waste from the blood. It's important because it helps assess kidney health, particularly to detect chronic kidney disease early on.

What substances can be used to estimate the GFR?

The GFR can be estimated using substances such as inulin, creatinine, and cystatin C. These substances help gauge how efficiently the kidneys filter blood.

How does the filtration process work in the kidney?

In the kidney, filtration is the process where plasma (containing waste) enters the nephron from the blood. Reabsorption returns useful substances like glucose back into the blood, while excretion removes waste in the urine. Secretion is when additional waste is added to the urine.

What is the filtration fraction and how is it calculated?

The filtration fraction is the percentage of renal plasma flow that is filtered through the glomerulus. It is calculated as the ratio of glomerular filtration rate (GFR) to renal plasma flow (RPF), typically about 20%.

What is the normal value of GFR and how does it change with age?

A normal GFR is around 125 mL/min. As a person ages, GFR naturally declines by about 6.5 mL/min per decade starting from age 20.

Why do males tend to have a higher GFR than females?

Males typically have a higher GFR than females because they have a greater body surface area and muscle mass. These factors contribute to higher creatinine production, which in turn leads to higher GFR readings.

How can creatinine be used to estimate GFR and what is its limitation?

Creatinine is a waste product that is filtered by the kidneys, and its concentration in the blood can be used to estimate GFR. However, creatinine is partially secreted by the kidneys, which may lead to an overestimation of GFR by about 10%. This can be corrected with calculations.

What role does inulin play in measuring GFR?

Inulin is a substance that is freely filtered by the kidneys but neither reabsorbed nor secreted, making it an ideal substance for accurately measuring GFR. The amount of inulin administered to the body is equal to the amount excreted in the urine, allowing for precise GFR calculation.

What are the main factors affecting GFR?

Factors affecting GFR include renal blood flow, cardiac output, age, gender, muscle mass, and the presence of kidney disease. Blood pressure and the health of the glomerular filtration barrier also play significant roles.

What is cystatin C, and how does it relate to GFR measurement?

Cystatin C is a protein produced by all nucleated cells and is freely filtered by the kidneys. It is used as a marker for GFR because its concentration in the blood reflects kidney function. Unlike creatinine, cystatin C is not affected by muscle mass.