Chronic Kidney Disease (CKD) is a progressive, long-term condition characterized by a gradual decline in the kidneys’ ability to filter waste and fluid from the blood. The disease is defined by either evidence of kidney damage or a reduced filtering capacity that persists for at least three months. Since early stages are often without noticeable symptoms, a precise classification system is required to assess severity, predict patient outcomes, and guide medical intervention.
The Two Pillars of CKD Classification: GFR and Albuminuria
Medical professionals classify CKD severity using two primary measurements: the Glomerular Filtration Rate (GFR) and the level of albuminuria. GFR is a direct measure of how well the kidneys are filtering the blood, providing the best estimate of residual kidney function. This rate is estimated (eGFR) using a blood test that measures creatinine, a waste product, along with the patient’s age and sex.
Albuminuria refers to the presence of the protein albumin in the urine, which signals damage to the kidney’s filtering units (glomeruli). Healthy kidneys prevent albumin from passing into the urine. This measurement is determined by the Albumin-to-Creatinine Ratio (ACR) from a urine sample.
Albuminuria Categories
Category A1 represents a normal to mildly increased level, defined as less than 30 milligrams of albumin per gram of creatinine (mg/g). Category A2 signifies a moderately increased level, ranging from 30 to 300 mg/g, and indicates established kidney damage. Category A3, or severely increased albuminuria, is diagnosed when the ACR exceeds 300 mg/g.
Decoding the Five Stages of Kidney Function (G1 through G5)
The GFR measurement is used to assign one of five “G” stages, which categorize the degree of function loss. The GFR is measured in milliliters per minute per 1.73 square meters of body surface area (mL/min/1.73 m²). A CKD diagnosis in Stages G1 or G2 requires accompanying evidence of kidney damage, such as albuminuria, since these GFR ranges can sometimes be considered normal, especially for older adults.
Stage G1 and G2 (Normal to Mildly Decreased)
Stage G1 is defined by a GFR of 90 or higher, which is considered normal or high function. Stage G2 represents a mildly decreased function, with a GFR falling between 60 and 89.
Stage G3 (Moderately Decreased)
Stage G3 is separated into two subcategories due to differences in risk and management. Stage G3a indicates a mildly to moderately decreased GFR, spanning from 45 to 59. Stage G3b signifies a moderately to severely decreased function, with a GFR between 30 and 44. This distinction is clinically relevant because G3b patients face a higher risk of progression to kidney failure and complications than those in G3a.
Stage G4 and G5 (Severely Decreased to Failure)
A GFR between 15 and 29 classifies the condition as Stage G4, representing a severely decreased function. Patients in this stage are typically prioritized for specialist care and require preparation for kidney replacement therapies. The final classification, Stage G5, is defined as kidney failure. This occurs when the GFR drops below 15 or when the patient is already receiving dialysis.
How Classification Directs Monitoring and Treatment Strategies
The combined GFR and Albuminuria categories create a precise system, known as CGA classification, which directly informs the patient’s monitoring schedule and treatment plan. This combined grid places patients into different risk categories, ranging from low-risk green to very high-risk red. A patient’s placement on this grid dictates how frequently their kidney function should be measured, ensuring timely intervention.
For example, a G3a/A1 patient (mildly decreased GFR but normal albuminuria) might require annual monitoring, reflecting a lower risk profile. Conversely, a G3b/A3 patient (lower GFR combined with severely increased albuminuria) may require blood and urine tests every one to three months due to their high risk of disease progression. This focused monitoring ensures complications are caught early and treatments are adjusted promptly.
The classification also triggers the initiation of specific medical treatments and specialist referrals. Patients with an eGFR below 30 (Stage G4 and G5) are generally referred to a nephrologist for co-management. Management strategies are intensified based on the albuminuria level. Higher albuminuria (A2 or A3) often prompts the use of certain blood pressure medications, such as ACE inhibitors or ARBs, to reduce protein leakage and protect the kidney. The classification also guides screening for common CKD complications, such as anemia and bone disorders, often recommended starting at Stage G3a.