The Glomerular Filtration Rate (GFR) measures how effectively the kidneys are cleaning the blood. It quantifies the amount of blood filtered by the glomeruli, the tiny filtering units within the kidneys, each minute. A reduced GFR indicates that the kidneys are not clearing waste products efficiently, which defines the severity of Chronic Kidney Disease (CKD). Establishing specific GFR thresholds allows healthcare providers to categorize the disease into stages for standardized monitoring and treatment planning.
How GFR is Measured
The actual measurement of GFR is intricate and time-consuming, typically reserved for research settings, often involving the injection of markers like iohexol or inulin. For routine clinical practice, an estimated GFR (eGFR) is calculated using a simple blood test that measures Creatinine levels. Creatinine is a waste product generated consistently from the normal breakdown of muscle tissue. Healthy kidneys filter creatinine out of the bloodstream, but as kidney function declines, the blood Creatinine level rises.
The eGFR calculation formula, such as the CKD-EPI (Chronic Kidney Disease Epidemiology Collaboration) equation, uses this Creatinine value. This formula incorporates demographic factors like age and sex because these variables naturally influence muscle mass and baseline Creatinine production. Major medical organizations are actively removing the historical race adjustment factor from these formulas to ensure greater accuracy and equity in kidney function assessment.
The Stages of Kidney Disease Based on GFR
The CKD classification system, established by organizations like KDIGO (Kidney Disease: Improving Global Outcomes), divides the disease into six GFR categories, or G stages, based on the eGFR value (mL/min/1.73 m²). Stage G1 signifies normal or high function (eGFR 90 or greater), but a CKD diagnosis requires other evidence of kidney damage. Function is mildly decreased in Stage G2 (eGFR 60 to 89).
Stage G3, representing moderate function loss, is divided into two substages to reflect differences in risk and management intensity. Stage G3a is mildly to moderately decreased (eGFR 45 to 59), while Stage G3b is moderately to severely decreased (eGFR 30 to 44). Patients enter Stage G4, indicating severe filtration reduction (eGFR 15 to 29). The final stage is Stage G5, defined as kidney failure, occurring when the eGFR falls below 15, signifying near-total loss of filtering capacity.
Refining CKD Classification with Albuminuria
Relying solely on GFR can underestimate the true risk of disease progression, especially in the earlier stages. Therefore, current guidelines mandate the simultaneous assessment of Albuminuria, which is the presence of the protein albumin in the urine, indicating damage to the kidney’s filtering membrane. Albuminuria is measured using the urine Albumin-to-Creatinine Ratio (ACR) and is categorized into three A stages:
- Stage A1 represents normal to mildly increased albuminuria (ACR below 30 mg/g).
- Stage A2 indicates moderately increased levels (ACR between 30 and 300 mg/g).
- Stage A3 signifies severely increased albuminuria (ACR greater than 300 mg/g).
Albuminuria is a powerful predictor of complications, including the rate of CKD progression and the likelihood of developing cardiovascular disease. By combining the GFR category (G stage) with the Albuminuria category (A stage), clinicians use a risk matrix to precisely stratify a patient’s prognosis. For example, a patient in G3a with A3 is at a much higher risk of disease progression than a patient in G3a with A1, directly influencing the intensity of medical intervention.
Clinical Management and Monitoring by Stage
Clinical management is tailored directly to the patient’s G and A stage classification, focusing on mitigating risk factors and slowing the decline of kidney function.
Early Stages (G1-G3a)
For individuals in the early stages, the primary focus involves aggressive control of blood pressure and blood sugar, especially in those with diabetes or hypertension. Lifestyle modifications, such as dietary changes and increased physical activity, are recommended to manage cardiovascular risk.
Advanced Stages (G3b-G5)
As the disease progresses into Stage G3b and G4, monitoring frequency increases, and referral to a nephrologist becomes routine to manage the complexity of care. At these stages, complications like anemia and mineral and bone disorders (CKD-MBD) must be actively screened for and managed. When the eGFR approaches the Stage G5 range (typically below 20), preparation for Kidney Replacement Therapy (KRT) is initiated. This preparation includes discussing dialysis access and potential kidney transplantation to ensure a smooth transition to life-sustaining treatment.