IgA nephropathy: understanding glomerular filtration rate (GFR)

Immunoglobulin A nephropathy (IgAN) is a chronic disease that causes inflammation and damage to the kidneys, and a gradual loss of kidney function. One of the most important tests used to diagnose and monitor IgAN and other types of kidney disease is glomerular filtration rate (GFR). Below is a look at what this test measures and why it matters for people with kidney disease.

How the kidneys filter the blood

To understand glomerular filtration rate, it helps to review the basics of what the kidneys are and how healthy kidneys function.

The kidneys are a pair of organs located in the back, just below the bottom ribs. Each is about the size of a fist. The kidneys act as filters for the blood.

As blood moves through the kidneys, excess fluid, cellular waste, and other unneeded substances are removed and become urine that can be discarded from the body. This continuous process helps the blood maintain a balance of fluids, salts, minerals, proteins, and many other substances the body needs to function normally and remain healthy.

Nephrons

Each kidney contains approximately one million filtering units called nephrons:

• Each nephron has two main parts, the glomerulus and the tubule.
• The glomerulus is a network of small blood vessels. These blood vessels are the structure that filter out fluid, waste, and other substances as blood moves through the kidney. The plural of glomerulus is glomeruli.
• Fluid and substances that are removed from the blood enter a small tube called the tubule. The tubule acts as a kind of intersection for fluid and other substances that are filtered out of the kidneys. Some fluid and substances are absorbed back into the blood. Excess fluid and waste become urine and travel to the bladder.

IgA nephropathy, as well as many other kidney diseases, cause damage to the glomeruli. Over time, there are fewer functioning glomeruli and fewer functioning nephrons.

Glomerular filtration rate

Glomerular filtration rate (GFR) is how much blood the kidneys filter in a minute. A healthy kidney will filter between 90 milliliters and 120 milliliters of blood every minute, though this number varies with age, and normal GFR appears to slow down with increasing age.

When there is damage to the kidneys, the kidneys cannot filter blood as quickly or as effectively. Measuring GFR determines how well the kidneys are functioning.

There are two main approaches to measuring GFR:

Direct measurement of GFR

This test is more accurate, but it is less commonly used. It requires a complex process that is only available in specialized medical centers.

During this test, a person will be given an infusion into the bloodstream containing a marker. A marker is a substance that does not occur naturally in the body and will not be absorbed or broken down by the body. Depending on the type of marker used, a person will give samples of blood and urine at designated times. These samples will be tested for the marker. The idea is to track how long it takes for kidneys to clear the marker from the body.

Estimated GFR (eGFR)

Because a direct measure of GFR is complex, it is more common to use tests that provide an estimated GFR (eGFR).

Estimated GFR is calculated by measuring the amount of a substance called creatinine in the blood and/or urine. Creatinine is a waste product created when the body digests proteins. It is also created by the normal breakdown of muscle tissue. (Creatinine should not be confused with creatine, which is a substance that supplies the muscles with energy).

The kidneys clear creatinine from the blood. Higher than normal levels of creatinine in the blood and urine can be a sign of kidney disease, and it can be used to estimate GFR. Normal levels can vary from person to person based on age and body composition.

A test called a serum creatinine blood test is most commonly used. This test requires a healthcare provider to take a blood sample. The blood sample is tested in a lab, and an equation is used to determine how well the kidneys are clearing creatinine from the body.

A test called a creatinine clearance test can also be used to calculate an eGFR. This test requires a person to collect urine over a period of 24 hours. A healthcare provider will provide detailed instructions for the collection process, and following instructions exactly will ensure a more accurate test result. A creatinine clearance test also includes bloodwork to test for creatinine levels in the blood.

Other important tests for kidney disease

GFR and eGFR are not the only tests that are used when diagnosing and monitoring IgAN and other forms of kidney disease. Diagnosis and assessment can involve additional blood tests, urine tests, imaging tests, kidney biopsy, and an overall assessment of health, including blood pressure, liver function, and coexisting health conditions.

Your healthcare provider will be your best source of information on what tests are recommended and what tests results mean.