Kidney disease is diagnosed primarily through two simple tests: a blood test that estimates how well your kidneys filter waste, and a urine test that checks for protein leaking into your urine. These two measurements, often done together during routine bloodwork, can catch kidney problems years before you feel any symptoms. Depending on the results, your doctor may order imaging or, in some cases, a biopsy to pinpoint the exact cause.
The Two Core Screening Tests
The foundation of kidney disease diagnosis comes down to two numbers: your estimated glomerular filtration rate (eGFR) and your urine albumin-to-creatinine ratio (uACR). Together, they tell your doctor how efficiently your kidneys are filtering blood and whether they’re letting protein slip through that should stay in your body.
Your eGFR is calculated from a standard blood draw that measures creatinine, a waste product your muscles produce at a fairly steady rate. Healthy kidneys clear creatinine efficiently, so when levels rise in your blood, it signals that your kidneys are struggling. The current formula used in the U.S., adopted in 2021, no longer factors in race. It uses your creatinine level, age, and sex to estimate what percentage of normal filtering capacity your kidneys still have. An eGFR of 90 or above is normal. Between 60 and 89 may indicate early damage, especially if other signs are present. Below 60 sustained over three months is the threshold for a chronic kidney disease (CKD) diagnosis.
The uACR checks for albumin, a protein that healthy kidneys keep in your blood. To get this number, you simply provide a urine sample. Less than 30 milligrams of albumin per gram of creatinine is normal. Between 30 and 300 is considered moderately increased albuminuria, sometimes still called “microalbuminuria,” and it’s one of the earliest detectable signs of kidney damage. Above 300 indicates more significant protein loss, pointing to advancing disease. Because albumin levels in urine can fluctuate day to day, doctors typically want to see abnormal results on at least two out of three samples before confirming a diagnosis.
Who Should Be Tested, and How Often
Kidney disease rarely causes noticeable symptoms until it’s advanced, which makes routine screening essential for people at higher risk. The National Kidney Foundation and the American Diabetes Association both recommend annual testing with both eGFR and uACR for anyone with diabetes. People with high blood pressure, heart disease, a family history of kidney failure, or obesity also benefit from regular screening. If you already have mildly elevated albumin levels, guidelines suggest testing one to three times per year depending on how much protein is showing up.
Additional Blood Markers
Creatinine isn’t the only blood value that signals kidney trouble. Blood urea nitrogen (BUN) is another waste product your kidneys normally clear. When BUN rises alongside creatinine, it reinforces that kidney function is declining rather than the creatinine bump being caused by something else, like a high-protein meal or intense exercise.
A comprehensive metabolic panel, which is standard bloodwork, also reveals electrolyte imbalances that damaged kidneys can cause. Potassium, phosphorus, and calcium levels all shift as kidney function drops, and tracking them helps your doctor understand how much the disease is affecting your body’s chemistry.
When Standard Creatinine Isn’t Reliable
Creatinine-based eGFR works well for most people, but it has a blind spot: it depends on muscle mass. If you have unusually high or low muscle mass, your creatinine levels won’t accurately reflect your kidney function. This matters for older adults who’ve lost significant muscle, people who are very obese, those with paralysis, or anyone who is malnourished.
In these situations, doctors can order a cystatin C test. Cystatin C is a small protein produced by nearly all cells in the body at a steady rate, and its blood levels aren’t meaningfully affected by muscle mass, age, sex, or race. It also responds faster than creatinine when kidney function changes suddenly, making it more accurate for hospitalized patients whose kidneys may be deteriorating in real time. The most precise eGFR estimate combines both creatinine and cystatin C, though cystatin C testing isn’t yet routine at most labs.
Imaging Tests
Blood and urine tests reveal how well your kidneys work, but they don’t show why. Imaging fills that gap. An ultrasound is typically the first step because it’s noninvasive, uses no radiation, and gives a clear picture of kidney size, shape, and structure. Kidneys that appear shrunken suggest long-standing chronic disease. Enlarged kidneys can point to polycystic kidney disease or acute inflammation. Ultrasound also detects blockages like kidney stones or tumors pressing on the urinary tract.
CT scans offer more anatomical detail and are sometimes used when ultrasound findings are unclear. For kidney stones specifically, a large trial published in the New England Journal of Medicine found that ultrasound and CT had similar rates of confirmed diagnoses at six months (roughly 32 to 35 percent across both groups). Ultrasound had slightly lower sensitivity but higher specificity, meaning it missed a few more stones but was less likely to flag something that wasn’t there. Since CT involves radiation exposure, many doctors start with ultrasound and reserve CT for cases where more precision is needed.
Kidney Biopsy
When blood tests, urine tests, and imaging point to kidney disease but the underlying cause remains unclear, a biopsy provides a definitive answer. This is especially common when doctors suspect conditions like glomerulonephritis (inflammation of the kidney’s filtering units), lupus-related kidney damage, or unexplained rapid decline in kidney function.
During a percutaneous kidney biopsy, you lie face down with a pillow supporting your abdomen. After numbing the skin with a local anesthetic and sometimes giving mild sedation through an IV, the doctor uses ultrasound to guide a thin needle into your kidney. The needle has a spring-loaded trigger that makes a clicking or popping sound as it quickly enters and exits the kidney tissue. You’ll be asked to hold your breath for a few seconds during each pass, and the doctor may insert the needle more than once to collect enough tissue. The whole procedure takes about an hour, and no stitches are needed afterward, just a bandage.
Recovery involves lying in a monitored room for several hours while staff checks your blood pressure, pulse, and urine for signs of internal bleeding. Most people go home the same day, though an overnight stay is sometimes necessary. Plan on avoiding heavy lifting, contact sports, and other strenuous activity for about two weeks. The tissue samples are examined under a microscope, which can reveal the specific type of kidney disease, how much damage has already occurred, and how likely it is to respond to treatment.
Diagnosing Acute Kidney Injury
Not all kidney disease develops slowly. Acute kidney injury (AKI) can happen over hours or days, often during a hospital stay triggered by severe infection, dehydration, or medication reactions. The diagnostic criteria are straightforward: AKI is identified when serum creatinine rises by 0.3 mg/dL or more within 48 hours, or climbs to at least 1.5 times its baseline within seven days, or urine output drops below 0.5 mL per kilogram of body weight per hour for six hours.
Severity is graded in three stages. Stage 1 corresponds to that initial creatinine bump of 1.5 to 1.9 times baseline. Stage 2 means creatinine has doubled or nearly tripled. Stage 3, the most severe, involves creatinine tripling or exceeding 4.0 mg/dL, urine output dropping to near zero, or the need to start dialysis. These criteria help hospital teams act quickly, because AKI caught early is often reversible, while delayed recognition can lead to permanent damage.
How the Stages of CKD Are Determined
Once chronic kidney disease is confirmed, staging tells you and your doctor how much function remains. There are five stages, defined almost entirely by eGFR:
- Stage 1 (eGFR 90+): Normal filtering rate, but other evidence of damage exists, such as protein in the urine or structural abnormalities on imaging.
- Stage 2 (eGFR 60–89): Mildly reduced function with signs of damage.
- Stage 3 (eGFR 30–59): Moderately reduced function. This is the stage where symptoms like fatigue or swelling may first appear.
- Stage 4 (eGFR 15–29): Severely reduced function. Planning for possible dialysis or transplant typically begins here.
- Stage 5 (eGFR below 15): Kidney failure. The kidneys can no longer sustain life without dialysis or a transplant.
Staging also considers how much albumin is in your urine, since two people with the same eGFR can have very different outlooks depending on whether protein loss is mild or severe. Together, these two values guide treatment decisions and help predict how quickly the disease is likely to progress.