Glomerular Filtration Rate (GFR) Test

The Glomerular Filtration Rate (GFR) test is a medical procedure used to assess the function of the kidneys by estimating the volume of filtered fluid through the glomeruli per unit time. The glomeruli are tiny blood vessels in the kidneys that filter waste and excess fluids from the blood.

Purpose

The primary purpose of the GFR test is to:

• Diagnose kidney disease or damage
• Monitor the progression of kidney disease
• Evaluate the effectiveness of treatment for kidney disease
• Assess the risk of kidney disease in people with certain medical conditions, such as diabetes or high blood pressure

Types of GFR Tests

There are several types of GFR tests, including:

• Inulin clearance test: This is considered the gold standard for measuring GFR, but it is rarely used due to its complexity and cost.
• Creatinine clearance test: This test measures the amount of creatinine in the blood and urine over a 24-hour period.
• Estimated GFR (eGFR) test: This test uses a formula to estimate GFR based on the level of creatinine in the blood, age, sex, and other factors.
• Cystatin C-based eGFR test: This test uses a protein called cystatin C to estimate GFR.

How the Test is Performed

The procedure for the GFR test varies depending on the type of test:

• Inulin clearance test: A catheter is inserted into a vein, and a solution containing inulin is infused. Blood and urine samples are collected over a period of time to measure the amount of inulin cleared from the blood.
• Creatinine clearance test: A 24-hour urine collection is required, during which time all urine is saved and sent to a laboratory for analysis. Blood samples are also taken at the beginning and end of the collection period.
• eGFR test: A blood sample is taken to measure the level of creatinine in the blood. The eGFR is then calculated using a formula that takes into account age, sex, and other factors.

Interpretation of Results

The results of the GFR test are used to diagnose and monitor kidney disease:

• Normal GFR: A GFR of 90 mL/min/1.73m² or higher is considered normal.
• Mild kidney disease: A GFR between 60-89 mL/min/1.73m² indicates mild kidney disease.
• Moderate kidney disease: A GFR between 30-59 mL/min/1.73m² indicates moderate kidney disease.
• Severe kidney disease: A GFR between 15-29 mL/min/1.73m² indicates severe kidney disease.
• Kidney failure: A GFR below 15 mL/min/1.73m² indicates kidney failure.

Risks and Limitations

The GFR test is generally a safe procedure, but there are some risks and limitations to consider:

• Blood sampling: There is a small risk of bleeding or bruising at the needle site.
• Urine collection: There is a risk of urinary tract infection if the urine collection is not done properly.
• Inaccurate results: The eGFR test may not be accurate in certain situations, such as in people with liver disease or those who are malnourished.