Kidney disease is staged using two lab values: your estimated glomerular filtration rate (eGFR), which measures how well your kidneys filter blood, and your urine albumin-to-creatinine ratio (ACR), which measures how much protein is leaking into your urine. Together, these two numbers place you into a specific stage and risk category that guides treatment decisions. The system has five main stages, with stage 1 representing near-normal function and stage 5 representing kidney failure.
The Two Numbers That Determine Your Stage
Your eGFR is calculated from a blood test that measures creatinine, a waste product your muscles produce at a fairly constant rate. When your kidneys work well, they clear creatinine efficiently. As kidney function declines, creatinine builds up, and your eGFR drops. The current standard formula, adopted in 2021, no longer uses race as a variable, a change the National Kidney Foundation and American Society of Nephrology recommended to reduce health disparities.
The second number, your ACR, comes from a urine sample. Healthy kidneys keep protein in your blood, so finding albumin (a type of protein) in your urine signals kidney damage. Even when your eGFR looks normal, a high ACR can indicate early disease. The ACR is measured in milligrams per gram of creatinine and falls into three categories:
- A1: Less than 30 mg/g (normal to mildly increased)
- A2: 30 to 300 mg/g (moderately increased)
- A3: Greater than 300 mg/g (severely increased)
For a formal diagnosis of chronic kidney disease, these abnormalities need to persist for at least three months. Guidelines recommend testing on three separate occasions over that period, with at least two of the three results coming back positive, to rule out temporary causes like dehydration or a urinary tract infection.
The Five Stages by eGFR
Each stage corresponds to a range of kidney filtration, measured in milliliters per minute. A normal eGFR is around 90 or above. Here’s how the stages break down:
- Stage 1 (G1): eGFR 90 or above. Kidney function is normal or high, but other signs of damage are present, such as protein in the urine, abnormal imaging, or a known genetic condition like polycystic kidney disease.
- Stage 2 (G2): eGFR 60 to 89. A mild decrease in filtration. Like stage 1, this only counts as CKD if there’s additional evidence of kidney damage.
- Stage 3a (G3a): eGFR 45 to 59. A mild-to-moderate decrease. This is often the stage where kidney disease is first caught on routine bloodwork.
- Stage 3b (G3b): eGFR 30 to 44. A moderate-to-severe decrease. Complications like anemia, bone disease, and fluid imbalances become more common here.
- Stage 4 (G4): eGFR 15 to 29. Severe reduction in function. Symptoms like fatigue, swelling, nausea, and changes in urination often appear.
- Stage 5 (G5): eGFR below 15. This is kidney failure. Most people at this stage need dialysis or a kidney transplant to survive.
Why Stage 3 Is Split in Two
Stage 3 covers a wide range of kidney function, and the difference between its two halves is clinically significant. People with stage 3b have roughly double the mortality rate of those with stage 3a: 4.8 deaths per 100 person-years compared to 1.1. Cardiovascular event rates also more than triple, jumping from 3.7 per 100 person-years in stage 3a to 11.3 in stage 3b. The split at an eGFR of 45 helps doctors identify which patients need more aggressive monitoring and treatment versus those who may remain stable for years with lifestyle management alone.
How the Risk Heat Map Works
Your stage number alone doesn’t tell the full story. The international guidelines group KDIGO developed a color-coded heat map that combines your eGFR stage (G1 through G5) with your albuminuria category (A1 through A3) to assign an overall risk level. The colors work like a stoplight:
- Green (low risk): G1 or G2 with A1 albuminuria. Kidney function is preserved and protein leakage is minimal.
- Yellow (moderate risk): G1 or G2 with A2, or G3a with A1. There’s either early protein leakage or mildly reduced filtration, but not both at concerning levels.
- Orange (high risk): G1 or G2 with A3, G3a with A2, or G3b with A1. At least one marker is significantly abnormal.
- Red (very high risk): G3a or worse with A3, G3b or worse with A2, or G4/G5 with any albuminuria level.
This combined approach predicts outcomes better than either number alone. Each step up on the heat map roughly doubles the risk of both disease progression and death, after adjusting for other health conditions. The heat map is the reason two people with the same eGFR can receive very different treatment plans: someone at G3a with heavy protein leakage (A3) is at higher risk than someone at G3a with no protein in their urine (A1).
When Creatinine Isn’t Reliable
The standard eGFR calculation depends on creatinine, which is tied to muscle mass. That makes it unreliable for certain people. If you have very low muscle mass, use a wheelchair, have a neuromuscular condition, or are undergoing chemotherapy, your creatinine may be artificially low, making your kidneys look healthier than they are.
In these cases, doctors can use a second blood marker called cystatin C. Unlike creatinine, cystatin C is produced by all cells in the body at a relatively constant rate regardless of muscle mass. KDIGO guidelines specifically recommend cystatin C testing for adults whose creatinine-based eGFR falls between 45 and 60 but who have no other signs of kidney damage. The cystatin C result can either confirm a CKD diagnosis or rule it out, avoiding unnecessary treatment.
How Staging Differs for Children
Children aren’t small adults when it comes to kidney function. Their creatinine levels are naturally lower because they have less muscle mass, and normal eGFR values change as they grow. Pediatric kidney disease is staged using a different formula called the bedside Schwartz equation, developed from the Chronic Kidney Disease in Children study. It uses just two inputs: the child’s height and their creatinine level. The same five-stage framework applies, but the calculation is calibrated to account for the fact that a creatinine level considered normal in a child might signal serious disease in an adult.
What Happens at Stage 5
Stage 5, defined by an eGFR below 15, is kidney failure. At this point, the kidneys can no longer clear enough waste and fluid to keep the body functioning safely. Symptoms that may have been subtle in earlier stages, like nausea, itching, loss of appetite, and severe fatigue, tend to become constant. Most people develop uremic symptoms once their eGFR drops below 30, and these intensify as function continues to decline.
Planning for dialysis or transplant typically begins well before stage 5. Referral for transplant evaluation generally happens when eGFR falls below 20, and if dialysis will be needed, surgical preparation for vascular access starts around the same threshold because the access site takes about three months to mature before it can be used. This lead time is why regular monitoring in stages 3 and 4 matters: it allows for preparation rather than emergency intervention.