The Glomerular Filtration Rate (GFR) is a measurement that assesses kidney function by estimating how much blood passes through the kidneys’ tiny filters, called glomeruli, each minute. The GFR is a primary indicator of overall kidney health. This measurement is useful for diagnosing and monitoring kidney disease, often before symptoms appear.
The Kidneys’ Filtration System
Each kidney contains about a million microscopic filtering units called nephrons, and each nephron contains a glomerulus. The glomerulus functions like a sieve, consisting of a network of tiny blood vessels through which blood flows under pressure. This allows certain substances to pass from the blood into what will become urine while retaining others.
As blood moves through the glomeruli, the pressure pushes water, waste products like urea and creatinine, and excess minerals out of the capillaries. This filtered fluid, known as filtrate, is the precursor to urine. The filtration barrier is selective, preventing larger components like red blood cells and proteins from leaving the bloodstream, ensuring waste is removed while preserving what the body needs.
In healthy kidneys, the glomeruli filter a significant volume of blood. The vast majority of this filtered fluid is then reabsorbed back into the blood by other parts of the nephron. Only a small fraction, containing concentrated waste, becomes urine.
How Glomerular Filtration Rate Is Measured
The most common method for determining GFR is an estimation known as eGFR. This approach uses a routine blood test that measures creatinine, a waste product from the breakdown of muscle and dietary protein. Under normal circumstances, the kidneys filter creatinine from the blood at a relatively steady rate.
When kidney function declines, less creatinine is filtered, causing its concentration in the blood to rise. To provide a more accurate assessment, the eGFR calculation incorporates other variables. These factors, which include a person’s age, sex, and body size, help account for differences in muscle mass and creatinine production.
While eGFR is standard, a more complex direct measurement called measured GFR (mGFR) can also be performed. This procedure is more time-consuming, often involving a 24-hour urine collection, and is reserved for specific situations where a highly precise measurement is needed.
Interpreting GFR Results
GFR results are expressed in milliliters per minute (mL/min) and are used to classify kidney function. For most healthy adults, a GFR of 90 mL/min or higher is normal. GFR naturally declines with age, so an older individual may have a lower GFR without having kidney disease; for example, the average eGFR for a person in their 20s is 116, while for someone in their 60s, it is closer to 85.
A GFR result below 60 mL/min for three months or more is an indicator of chronic kidney disease (CKD). Healthcare providers use a staging system based on GFR levels to classify the severity of CKD. Stage 1 CKD is diagnosed when a person has a GFR of 90 or higher but also has other signs of kidney damage, like protein in the urine. Stage 2 CKD is characterized by a GFR between 60 and 89, indicating a mild reduction in kidney function.
As the GFR number decreases, the severity of kidney disease increases. Stage 3 CKD is divided into Stage 3a (GFR 45-59) and Stage 3b (GFR 30-44), representing a mild-to-moderate and moderate-to-severe decline. Stage 4 CKD is defined by a GFR between 15 and 29, indicating a severe reduction in function. A GFR below 15 signifies Stage 5 CKD, or kidney failure, where the kidneys can no longer adequately filter waste.
Causes and Management of Low GFR
A persistently low GFR is often the result of chronic conditions that damage the kidneys. The most common causes are diabetes and high blood pressure (hypertension), which can harm the delicate blood vessels in the glomeruli. Other causes of a low GFR include:
- Autoimmune diseases, such as lupus
- Polycystic kidney disease
- Recurrent kidney infections
- Long-term use of certain medications, like nonsteroidal anti-inflammatory drugs (NSAIDs)
- Acute kidney injury from severe dehydration, major blood loss, or a urinary tract blockage
Management of a low GFR focuses on treating the underlying cause and slowing the progression of kidney damage. For those with diabetes or hypertension, this involves controlling blood sugar and blood pressure through medication, diet, and lifestyle changes. A kidney-friendly diet that limits sodium, phosphorus, and protein can also reduce the workload on the kidneys. Staying hydrated and reviewing all medications with a healthcare provider is also recommended.