Chronic kidney disease (CKD) is diagnosed when blood tests, urine tests, or imaging show abnormal kidney function or damage that has persisted for at least three months. The two core markers are your estimated glomerular filtration rate (eGFR), which measures how well your kidneys filter blood, and the amount of protein leaking into your urine. Either one below normal for three or more months is enough for a diagnosis.
The Two Numbers That Define CKD
Your doctor is looking at two things: how fast your kidneys are filtering waste, and whether they’re leaking protein they shouldn’t be. These correspond to two simple tests.
The first is eGFR, calculated from a blood draw that measures creatinine (a waste product from muscle metabolism). A normal eGFR is 90 or above. Anything below 60 for three months or longer qualifies as CKD on its own, even without other signs of damage. The second is a urine albumin-to-creatinine ratio (UACR), which checks for a protein called albumin in your urine. A result above 30 mg/g means albumin is leaking through your kidneys, a sign of damage. Levels between 30 and 300 mg/g are considered moderately increased, while anything above 300 mg/g is severely increased. Either an eGFR below 60 or a UACR above 30, sustained over three months, leads to a CKD diagnosis.
How eGFR Determines Your Stage
Once CKD is confirmed, your eGFR number tells your doctor how much kidney function you’ve lost. The staging system runs from 1 to 5:
- Stage 1 (eGFR 90+): Normal filtration, but other evidence of kidney damage exists, like protein in your urine.
- Stage 2 (eGFR 60–89): Mildly reduced function with signs of damage.
- Stage 3a (eGFR 45–59): Mild to moderate loss of function.
- Stage 3b (eGFR 30–44): Moderate to severe loss.
- Stage 4 (eGFR 15–29): Severe loss of function.
- Stage 5 (eGFR below 15): Kidney failure.
Stages 1 and 2 require evidence of kidney damage beyond the eGFR number itself, since mild drops in filtration rate can be normal, especially with aging. Stage 3 and beyond are diagnosed on eGFR alone.
What the Blood Test Involves
The standard blood test measures serum creatinine, which your body produces at a fairly steady rate from normal muscle activity. When your kidneys aren’t filtering well, creatinine builds up in the blood. A lab formula then converts your creatinine level into an eGFR using your age and sex.
The current standard formula, adopted in 2021, no longer uses race as a variable. Previous equations adjusted the result based on whether a patient was Black, but the updated version eliminated that factor. However, the creatinine-only formula can still be off by about 4% in either direction depending on the population. For more precision, especially when results are near a cutoff that would change treatment decisions, doctors can add a second blood marker called cystatin C.
Cystatin C is produced by nearly all cells in the body at a constant rate, and its blood level isn’t significantly affected by muscle mass, age, sex, or race. That makes it more reliable for people whose creatinine might be misleading: older adults with low muscle mass, people who are very muscular, malnourished individuals, or amputees. Using both creatinine and cystatin C together gives the most accurate eGFR estimate.
If your test is part of a broader metabolic panel, you may need to fast for up to 12 hours beforehand. Even for a standalone creatinine test, you may be asked to avoid eating meat for 24 hours, since meat can temporarily raise creatinine levels.
What the Urine Test Shows
Healthy kidneys keep large proteins like albumin in the bloodstream. When the filtering units are damaged, albumin slips through into urine. A standard dipstick test catches large amounts, but the UACR test is more sensitive. It can detect smaller leaks (sometimes called microalbuminuria) between 30 and 300 mg/g that a dipstick would miss entirely.
A single abnormal UACR doesn’t mean CKD. Temporary spikes can happen after vigorous exercise, during a urinary tract infection, or with dehydration. Your doctor will repeat the test to confirm the result persists before making a diagnosis. That three-month persistence rule is central to distinguishing CKD from a temporary problem.
Imaging and Further Testing
A kidney ultrasound is often ordered alongside blood and urine work. It gives your doctor a picture of kidney size, shape, and internal structure. In CKD, the kidneys typically appear smaller than normal and the outer layer (the cortex) looks brighter than the surrounding tissue on the scan, a sign of scarring. The ultrasound can also reveal cysts, blockages, or structural abnormalities that might explain the kidney damage.
Small kidneys on ultrasound are one of the strongest visual clues that kidney disease is chronic rather than a sudden problem. Some forms of CKD are exceptions, though. Diabetic kidney disease, polycystic kidney disease, and certain infiltrative conditions can cause kidneys to appear normal or even enlarged despite ongoing damage.
In some cases, a kidney biopsy is needed to identify the specific cause of damage. This is more common when blood and urine patterns don’t clearly point to a cause, or when a treatable condition like a specific type of inflammation is suspected.
How CKD Is Told Apart From Acute Kidney Injury
When blood tests first reveal reduced kidney function, one of the most important questions is whether the problem is chronic or acute. Acute kidney injury (AKI) comes on quickly, often over days, and can be reversible. CKD develops gradually over months or years.
Several clues help distinguish them. The most reliable is documentation showing reduced eGFR for three months or more. Small kidneys on ultrasound strongly suggest CKD. Anemia, high phosphorus levels, and low calcium also point toward a chronic process, since these develop as the kidneys gradually lose their ability to produce hormones and regulate minerals. Chronic symptoms like fatigue, nausea, itching, and waking up at night to urinate suggest the problem has been building for a while. By contrast, rapidly rising creatinine levels from day to day, or sudden drops in urine output, are more characteristic of AKI.
Who Should Be Tested and How Often
CKD often causes no symptoms until it’s advanced, which is why screening matters for people at higher risk. If you have diabetes, screening is recommended by the American Diabetes Association. If you’re being treated for high blood pressure, multiple guidelines also recommend regular kidney checks, since sustained high blood pressure gradually damages the tiny blood vessels in the kidneys.
Other risk factors that warrant screening include a family history of kidney disease, heart disease, obesity, and being over 60. If you fall into one of these groups and haven’t had your eGFR or UACR checked recently, it’s a routine addition to standard blood work. For people with diabetes or hypertension who have normal initial results, annual screening is typical. If early-stage CKD is found, your doctor will test more frequently to track whether function is stable or declining.