The Glomerular Filtration Rate (GFR)

The Glomerular Filtration Rate (GFR), how it is measured, and the estimated glomerular filtration rate (eGFR).

The glomerular filtration rate (GFR) is a measure of how well the kidneys are filtering waste products from the blood. It is an important indicator of kidney function and is used to diagnose and monitor a range of kidney diseases. 

What is Glomerular Filtration Rate (GFR)?

The glomerular filtration rate (GFR) is the rate at which blood is filtered through the glomeruli, the tiny blood vessels in the kidneys that help remove waste products from the body. The GFR is typically measured in milliliters per minute (mL/min) and is an important indicator of how well the kidneys are functioning.

Values of Glomerular Filtration Rate (GFR)

The GFR is measured in units of mL/min/1.73m2, which takes into account the person's body surface area.

The normal GFR range is generally considered to be between 90 and 120 mL/min/1.73m2. A GFR below 60 mL/min/1.73m2 for more than 3 months is an indication of chronic kidney disease (CKD). CKD is classified into stages based on the GFR values, as follows:

Stage 1: GFR ≥ 90 mL/min/1.73m2 with other evidence of kidney damage

Stage 2: GFR 60-89 mL/min/1.73m2 with other evidence of kidney damage

Stage 3: GFR 30-59 mL/min/1.73m2

Stage 4: GFR 15-29 mL/min/1.73m2

Stage 5: GFR < 15 mL/min/1.73m2 (also known as end-stage kidney disease)

It is important to note that GFR values can vary depending on age, gender, race, muscle mass, and other factors. Additionally, GFR should be interpreted in the context of other clinical and laboratory parameters, including serum creatinine levels, urine output, proteinuria, and electrolyte levels.

How is GFR Measured?

GFR is typically measured using a blood test and a urine test. The blood test measures the level of creatinine in the blood, which is a waste product that is normally filtered out by the kidneys. If the creatinine level in the blood is high, it may indicate that the kidneys are not functioning properly.

The urine test measures the amount of creatinine in the urine over a period of 24 hours. This test provides an accurate measurement of how much creatinine is being filtered out of the blood by the kidneys.

The GFR is then calculated using a formula that takes into account the creatinine level in the blood, the age, sex, and race of the person, and other factors that may affect kidney function. The most commonly used formula to calculate GFR is the Modification of Diet in Renal Disease (MDRD) equation.

The Modification of Diet in Renal Disease (MDRD) equation

The Modification of Diet in Renal Disease (MDRD) equation is a formula used to estimate the glomerular filtration rate (GFR) based on the level of creatinine in the blood, the age, sex, and race of the person, and other factors that may affect kidney function. The MDRD equation is one of the most commonly used formulas to calculate GFR and is widely used in clinical practice and research.

The MDRD equation takes into account the following variables:

• Creatinine level in the blood

• Age

• Sex

• Race (African American or not)

• Serum albumin level (a protein in the blood)

• Serum blood urea nitrogen level (a waste product in the blood)

The MDRD equation is as follows:

GFR = 175 x (serum creatinine)^-1.154 x (age)^-0.203 x (0.742 if female) x (1.212 if African American) x (serum albumin)^-0.207 x (blood urea nitrogen)^0.711

In this equation, serum creatinine is measured in milligrams per deciliter (mg/dL), age is measured in years, serum albumin is measured in grams per deciliter (g/dL), and blood urea nitrogen is measured in milligrams per deciliter (mg/dL).

The MDRD equation provides an estimated GFR (eGFR) in units of mL/min/1.73m2, which takes into account the person's body surface area. The eGFR is used to classify kidney disease into stages, ranging from stage 1 (mild kidney disease) to stage 5 (end-stage kidney disease).

It is important to note that the MDRD equation has some limitations and may not be accurate in certain populations, such as those with extremes of body size or muscle mass. In these cases, other equations or methods may be used to estimate GFR, such as the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation or cystatin C-based equations.

What is the Estimated Glomerular Filtration Rate (eGFR)?

The estimated glomerular filtration rate (eGFR) is a calculation of GFR that is based on a person's age, sex, race, and creatinine level in the blood. The eGFR is a useful tool for monitoring kidney function and diagnosing kidney disease, as it provides an estimate of how well the kidneys are filtering waste products from the blood.

The eGFR is calculated using the same formula as the GFR, but it is often reported automatically as part of a routine blood test. The eGFR is reported in units of mL/min/1.73m2, which takes into account the person's body surface area.

The eGFR is used to classify kidney disease into stages, ranging from stage 1 (mild kidney disease) to stage 5 (end-stage kidney disease). The eGFR is also used to monitor the progression of kidney disease over time and to help guide treatment decisions.

Conclusion

The glomerular filtration rate (GFR) is an important measure of kidney function and is used to diagnose and monitor a range of kidney diseases. GFR is typically measured using a blood test and a urine test, and it is calculated using a formula that takes into account the creatinine level in the blood, age, sex, and other factors. 

The estimated glomerular filtration rate (eGFR) is a useful tool for monitoring kidney function and diagnosing kidney disease, as it provides an estimate of how well the kidneys are filtering waste products from the blood. It is important to monitor GFR and eGFR regularly in people with kidney disease to help guide treatment decisions and prevent complications.