Low kidney function means your kidneys aren’t filtering waste and excess fluid from your blood as effectively as they should. Doctors measure this with a number called eGFR (estimated glomerular filtration rate), which represents the percentage of normal filtering capacity your kidneys still have. An eGFR below 60, sustained for three months or more, is the standard threshold for chronic kidney disease. The lower the number, the less work your kidneys are doing.
How Kidney Function Is Measured
The primary test is a simple blood draw that measures creatinine, a waste product your muscles produce at a fairly constant rate. Healthy kidneys filter creatinine out efficiently, so when it builds up in your blood, that signals reduced filtering. Normal creatinine ranges are 0.74 to 1.35 mg/dL for adult men and 0.59 to 1.04 mg/dL for adult women. Your lab uses that creatinine level along with your age, sex, and body size to calculate your eGFR.
A second important test checks for protein in your urine. Healthy kidneys keep protein in the bloodstream, so finding it in urine means the kidney’s filtering units are damaged. This is measured with a urine albumin-to-creatinine ratio (uACR). A normal result is below 30 mg/g. Results between 30 and 299 mg/g indicate early kidney damage and raise your risk of kidney failure, heart failure, and stroke. A result of 300 mg/g or higher, confirmed on a repeat test, points to more significant kidney disease.
The Five Stages of Kidney Disease
Kidney disease is grouped into five stages based on eGFR:
- Stage 1 (eGFR 90 or above): Kidney function is normal or near-normal, but there’s evidence of damage, such as protein in the urine.
- Stage 2 (eGFR 60 to 89): A mild drop in function, usually still without noticeable symptoms.
- Stage 3 (eGFR 30 to 59): Moderate loss. This is where symptoms like fatigue, weakness, or swelling in the hands and feet may start to appear.
- Stage 4 (eGFR 15 to 29): Severe loss. Waste products are accumulating and complications become more likely.
- Stage 5 (eGFR below 15): Kidney failure. The kidneys can no longer filter waste adequately, and dialysis or a transplant is typically needed.
A key thing to understand: most people have no symptoms in stages 1 through 3. Many people first learn about reduced kidney function from routine bloodwork, not because they felt sick. This is why the condition often progresses silently for years before it’s caught.
Why Kidney Function Drops
Diabetes and high blood pressure are by far the two most common causes. About 1 in 3 people with diabetes and 1 in 5 people with high blood pressure develop kidney disease. Both conditions damage the tiny blood vessels inside the kidneys over time, gradually destroying the filtering units (called nephrons). Each kidney has about a million nephrons, and once they’re lost, they don’t regenerate. The remaining nephrons work harder to compensate, which can accelerate further damage.
Other causes include heart disease, a family history of kidney failure, recurrent kidney infections, autoimmune conditions like lupus, long-term use of certain pain relievers, and structural problems present from birth. Obesity and smoking also raise risk independently.
What Low Kidney Function Feels Like
In earlier stages, it feels like nothing. That’s the difficult part. You can lose more than half your kidney function before any symptoms show up. When they do appear, they’re often vague enough to be dismissed as stress or aging: tiredness, trouble sleeping, difficulty concentrating.
As kidney function declines further, the symptoms become harder to ignore. Advanced kidney disease can cause nausea, vomiting, loss of appetite, persistent fatigue and weakness, swelling in the legs or around the eyes, shortness of breath (from fluid buildup in the lungs), itchy skin, and high blood pressure that’s difficult to control. Mental sharpness can decrease as waste products accumulate in the blood, a state sometimes called uremic fog.
Complications Beyond Filtering
Your kidneys do more than just remove waste. They also produce hormones, regulate blood pressure, and help maintain the balance of minerals in your body. When kidney function drops, these secondary roles suffer too.
Anemia is one of the most common complications. Healthy kidneys produce a hormone that tells your bone marrow to make red blood cells. Damaged kidneys produce less of this hormone, so your body makes fewer red blood cells and delivers less oxygen to your tissues. On top of that, the red blood cells that are produced tend to die faster than normal in people with kidney disease, and nutrient deficiencies in iron, vitamin B12, and folate can make the problem worse. The result is persistent fatigue, weakness, and feeling cold.
Bone and mineral problems are another major concern. The kidneys help regulate calcium and phosphorus levels. When they can’t do this effectively, phosphorus builds up in the blood and calcium is pulled from the bones, leading to weaker bones and a higher risk of fractures. Excess phosphorus can also deposit in blood vessels and soft tissues, contributing to cardiovascular risk.
How It’s Managed
The goal at every stage is to slow the rate of decline and protect what function remains. For most people, this starts with controlling the underlying cause. Keeping blood sugar in a healthy range if you have diabetes, and getting blood pressure under control, can significantly slow further kidney damage.
Diet plays a real role, though the specifics depend on your stage and lab results. There’s no single kidney diet that applies to everyone. In general, as kidney function decreases, your body becomes less able to handle excess sodium, potassium, phosphorus, and protein. Too much sodium raises blood pressure and causes fluid retention. Too much potassium, which healthy kidneys easily regulate, can become dangerous when filtering is impaired, potentially causing heart rhythm problems. Excess phosphorus contributes to the bone and cardiovascular issues described above. Protein is still essential, but in later stages, eating more than your body needs creates extra waste that struggling kidneys must process. A dietitian who specializes in kidney disease can help you find the right balance for your specific situation.
At stage 5, when the kidneys can no longer sustain life on their own, the options are dialysis or a kidney transplant. Dialysis takes over the filtering job, either through a machine that cleans the blood directly or through a process that uses the lining of your abdomen. A transplant replaces the failed kidney with a healthy one from a donor. Both are well-established treatments, and the choice between them depends on your overall health, lifestyle, and personal preferences.
What Your eGFR Number Actually Tells You
If you’ve gotten a lab result showing a low eGFR, it helps to think of it as a rough percentage. An eGFR of 45 means your kidneys are working at roughly 45% capacity. That’s significant, but it also means nearly half your filtering ability is still intact. Many people live for years or even decades at stage 3 without ever progressing to kidney failure, especially if the underlying cause is managed well.
A single low reading doesn’t necessarily mean chronic kidney disease. Dehydration, certain medications, a recent intense workout, or even a high-protein meal before the test can temporarily raise creatinine and lower your eGFR. That’s why doctors confirm the diagnosis with repeat testing over at least three months. If your numbers stay consistently low, that’s when the diagnosis becomes meaningful and a management plan takes shape.