Healthy kidneys filter about 50 gallons of blood every day, removing waste and excess fluid through urine. The most reliable way to know if yours are working properly is through two simple tests: a blood test that estimates how well your kidneys filter (called eGFR) and a urine test that checks for protein leaking into your urine. Most kidney problems develop silently, producing no symptoms until significant damage has already occurred, which is why testing matters more than how you feel.
The Two Tests That Matter Most
Your kidney function is primarily measured by a number called eGFR, or estimated glomerular filtration rate. This comes from a routine blood draw that measures creatinine, a waste product your muscles produce at a fairly steady rate. Healthy kidneys clear creatinine efficiently, so when it builds up in your blood, it signals that filtration is slowing down. A lab calculates your eGFR from that creatinine level, adjusted for your age and sex.
An eGFR of 60 or higher is considered normal. Below 60 suggests kidney disease, and 15 or lower points to kidney failure. These numbers represent a percentage of full function, so an eGFR of 45 means your kidneys are working at roughly 45% capacity.
The second key test checks your urine for albumin, a protein that healthy kidneys keep in the blood. A normal result is less than 30 mg/g on a urine albumin-to-creatinine ratio test. Anything above 30 mg/g can indicate kidney damage, even if your eGFR still looks normal. Because a single high reading can be a fluke, doctors typically confirm with a second test. Two elevated results over three months or more is considered a sign of kidney disease.
Other Blood Markers Your Doctor May Check
Blood urea nitrogen (BUN) is another waste product measured in routine blood panels. The normal range is 5 to 20 mg/dl, but this number is less precise than eGFR because so many things influence it. A high-protein diet can push BUN toward the upper end of normal without any kidney problem. Dehydration, fever, infections, and even gastrointestinal bleeding raise it. Conversely, a low-protein diet or liver disease can keep BUN artificially low. Doctors often look at BUN alongside creatinine rather than relying on it alone.
As kidney function declines, the kidneys also lose their ability to balance electrolytes. Potassium and phosphorus levels can climb in the blood, while calcium drops. High potassium is particularly dangerous because it can disrupt heart rhythm. These imbalances typically show up in later stages of kidney disease rather than early on, so normal electrolyte levels on a blood panel are reassuring but don’t rule out early damage.
Physical Signs That Something Is Wrong
Early kidney disease rarely causes symptoms you can feel. That’s the core challenge: by the time symptoms appear, the disease has often progressed significantly. As waste products build in the blood, you may notice fluid retention (swelling in the ankles, feet, or around the eyes), persistent fatigue, nausea, and vomiting. These are signs that the kidneys are struggling to keep up with daily filtration demands.
Some symptoms point to a more acute problem like an infection rather than chronic disease. Pain or burning during urination, a frequent urge to go, cloudy or reddish urine, fever, and pain in your back or side below the ribs can signal a bladder or kidney infection that needs prompt treatment. Foamy urine, which looks like the froth you’d see when scrambling eggs, can indicate excess protein leaking through damaged filters.
What an Ultrasound Can Reveal
If blood or urine tests raise concerns, your doctor may order a kidney ultrasound. This painless imaging test checks the size, shape, and internal structure of your kidneys. A healthy adult kidney measures between 10 and 12 centimeters long, roughly the size of a fist. The right kidney tends to be slightly smaller than the left, with a normal difference of no more than 1.5 to 2 centimeters between them.
Kidneys that are too small often indicate end-stage damage where functional tissue has been replaced by scarring. Kidneys that are unusually large can point to early-stage diabetic kidney disease, acute inflammation, or other conditions causing swelling. The ultrasound also looks at the internal texture of the kidney. In a healthy organ, the outer layer (cortex) has a distinct appearance compared to the inner structures. When that distinction blurs or the cortex appears unusually bright on the screen, it correlates with chronic kidney disease, scarring, or other damage.
The Five Stages of Kidney Disease
Kidney disease is classified into five stages based on eGFR, which helps doctors and patients understand how much function remains and what to expect.
- Stage 1 (eGFR above 90): Kidney function is normal or near-normal, but other signs of damage exist, such as protein in the urine. Most people feel completely fine.
- Stage 2 (eGFR 60 to 89): A mild decrease in filtration. Still no symptoms for most people, and many never progress beyond this stage.
- Stage 3 (eGFR 30 to 59): Moderate loss of function. Waste products may begin accumulating, and complications like high blood pressure or bone problems can develop.
- Stage 4 (eGFR 15 to 29): Severe loss of function. Symptoms become more likely, and planning for dialysis or transplant typically begins.
- Stage 5 (eGFR below 15): Kidney failure. The kidneys can no longer sustain life without dialysis or a transplant.
The key takeaway from this staging system is that stages 1 and 2 are often caught only through routine lab work. Waiting for symptoms means waiting until stage 3 or later, when treatment options narrow and the goal shifts from prevention to slowing further decline.
Who Should Get Tested Regularly
Certain conditions dramatically increase your risk. About 1 in 3 adults with diabetes has chronic kidney disease, making it the leading cause of kidney failure. High blood sugar damages the tiny blood vessels that form the kidney’s filtering units over time. Roughly 1 in 5 adults with high blood pressure also has kidney disease, because elevated pressure strains those same delicate vessels. Carrying excess weight raises your risk for both diabetes and high blood pressure, compounding the threat.
Beyond these major risk factors, a family history of kidney disease, a history of heart disease, and being over 60 all increase your likelihood of declining kidney function. If any of these apply to you, annual screening with a basic blood panel and urine albumin test gives you the earliest possible warning. These tests are inexpensive, widely available, and often already included in routine checkups for people managing diabetes or high blood pressure.
For people with no risk factors, kidney function is still checked as part of standard metabolic panels during routine physicals. If your eGFR is above 60, your urine albumin is below 30, and you have no symptoms, your kidneys are almost certainly doing their job.