The kidneys, two bean-shaped organs located on either side of your spine, perform many important roles. They filter waste products and excess water from the blood, produce urine, and maintain fluid and electrolyte balance.
When kidneys do not function properly, waste can build up, leading to various health issues. Assessing kidney function is important for overall health. The Glomerular Filtration Rate (GFR) is a primary measure used for this assessment.
Understanding GFR
GFR measures the rate at which your kidneys filter blood. It estimates how much blood passes through the glomeruli, tiny filters within the kidneys that remove waste products. A higher GFR indicates better kidney function, while a lower GFR suggests less efficient filtering.
GFR is typically estimated (eGFR) rather than directly measured due to the complexity of direct methods. The eGFR is commonly calculated using a blood test that measures creatinine, a waste product from normal muscle wear and tear. This creatinine level is combined with personal factors like age and sex in a mathematical formula. While creatinine is freely filtered by the glomerulus, small amounts are also actively secreted by peritubular capillaries, meaning creatinine clearance can slightly overestimate the true GFR by 10-20%.
Some calculations may also incorporate cystatin C, a protein produced by many cells. Cystatin C can offer a more accurate estimate of GFR as its levels are less affected by muscle size, age, or diet.
The GFR Classification Stages
The standardized classification system for Chronic Kidney Disease (CKD) is based on GFR levels, often following guidelines from organizations like Kidney Disease: Improving Global Outcomes (KDIGO). This system categorizes kidney disease into stages, helping healthcare providers understand the severity of kidney function decline. CKD is defined as kidney damage or a GFR below 60 mL/min/1.73 m² that persists for three months or more.
The classification includes five main stages, with Stage 3 further divided into two subcategories:
Stage 1: Kidney damage with normal or high GFR (90 mL/min/1.73 m² or above).
Stage 2: Kidney damage with a mild decrease in GFR (60 to 89 mL/min/1.73 m²).
Stage 3a: Moderate decrease in GFR (45 to 59 mL/min/1.73 m²).
Stage 3b: Moderate decrease in GFR (30 to 44 mL/min/1.73 m²).
Stage 4: Severe decrease in GFR (15 to 29 mL/min/1.73 m²).
Stage 5: Kidney failure, with GFR below 15 mL/min/1.73 m² or when a person is on dialysis.
What Each GFR Stage Means
Each GFR stage carries distinct implications for an individual’s health, reflecting the progression of kidney function decline.
Stage 1
Stage 1 indicates that while kidney function is normal or high, there are other signs of kidney damage, such as protein in the urine. At this early stage, healthcare providers focus on identifying the cause of kidney damage and managing underlying conditions like diabetes or high blood pressure to prevent further progression.
Stage 2
In Stage 2, there is a mild decline in kidney function. While symptoms may not be apparent, the disease is progressing. Regular monitoring becomes important to track the rate of decline and continue managing related health issues.
Stages 3a and 3b
In Stages 3a and 3b, kidney function has moderately declined. Individuals may begin to experience symptoms as toxins build up in the blood. Complications such as anemia and bone disease may start to emerge, requiring specific treatments alongside continued management of underlying conditions.
Stage 4
Stage 4 signifies a severe loss of kidney function, and symptoms are more pronounced, indicating a significant impact on daily life.
Stage 5
When GFR indicates Stage 5, it means kidney failure. The kidneys have lost most of their ability to filter waste and fluid. Individuals typically experience pronounced symptoms such as extreme tiredness, nausea, swelling, muscle cramps, and changes in urination frequency. Without intervention, such as dialysis or a kidney transplant, this stage can be life-threatening.
Factors Influencing GFR and Testing
Several factors can influence GFR levels and contribute to kidney disease. Age is a significant factor, as GFR naturally decreases with advancing age. Certain medical conditions are also strongly associated with reduced GFR and increased risk, including diabetes and high blood pressure. Uncontrolled diabetes can damage kidney filters due to high blood sugar, while high blood pressure can exert excessive force on the kidney’s delicate blood vessels.
Other risk factors include heart disease, a family history of kidney disease, being overweight or obese. Some medications, particularly long-term use of certain pain relievers like nonsteroidal anti-inflammatory drugs (NSAIDs), can also negatively impact kidney function. Additionally, conditions such as abnormal kidney structure, certain infections like hepatitis C or HIV, and specific autoimmune diseases can influence GFR.
Regular GFR testing is recommended, particularly for individuals with higher risk. Early detection allows for interventions to protect remaining kidney function and manage progression. Healthcare providers often include an estimated GFR calculation as part of routine blood tests for those with risk factors. Discussing GFR results helps individuals understand their kidney health and determine appropriate monitoring or management strategies.