Kidney disease is tested primarily through two simple lab tests: a blood test that estimates how well your kidneys filter waste, and a urine test that checks for protein leaking into your pee. Together, these two results can detect kidney disease even before you feel any symptoms. If those initial tests raise concerns, your doctor may follow up with imaging or, in rare cases, a biopsy to get a closer look.
The Blood Test: Estimating Your Filtration Rate
The core blood test for kidney disease measures a waste product called creatinine, which your muscles produce at a fairly steady rate. Healthy kidneys filter creatinine out of your blood efficiently. When kidney function drops, creatinine builds up, and that rise shows up in a standard blood draw.
Your lab doesn’t just report the raw creatinine number. It plugs it into a formula along with your age and sex to calculate something called your estimated glomerular filtration rate, or eGFR. This score tells you roughly how many milliliters of blood your kidneys clean per minute. A normal eGFR is 90 or above. Below 60 for three months or longer is one of the defining criteria for chronic kidney disease (CKD).
The current standard formula, called CKD-EPI, no longer includes a race-based adjustment. Both U.S. and U.K. guidelines removed that coefficient after reviews found it was not reliably accurate. The formula can also incorporate a second blood marker called cystatin C, which is less influenced by muscle mass and diet than creatinine. Your doctor may order cystatin C if you have unusually high or low muscle mass, are on a high-protein diet, or if your creatinine-based eGFR seems inconsistent with your overall health picture.
The Urine Test: Checking for Protein Leakage
Healthy kidneys keep proteins in your blood where they belong. When the kidney’s filtering units are damaged, a protein called albumin starts slipping through into your urine. The standard test for this is the urine albumin-to-creatinine ratio, or UACR. You provide a single urine sample (usually first thing in the morning), and the lab calculates the ratio.
A UACR above 30 mg/g signals albuminuria and counts as a marker of kidney damage. Levels between 30 and 300 mg/g represent moderate albumin leakage, sometimes called microalbuminuria, which a basic urine dipstick can miss entirely. Levels above 300 mg/g indicate more significant damage. Because the UACR is a ratio, it corrects for how concentrated or dilute your urine happens to be that day, making it more reliable than a simple dipstick.
This test is especially important for catching kidney disease early. You can have a normal eGFR (90 or above) and still have kidney disease if your UACR is elevated. That’s why guidelines define CKD as either an eGFR below 60 or a UACR of 30 mg/g or higher, persisting for at least three months.
Other Blood Markers Your Doctor May Check
A renal function panel often includes blood urea nitrogen, or BUN. Like creatinine, BUN is a waste product your kidneys clear. The ratio of BUN to creatinine can help distinguish between kidney damage and other causes of elevated waste products. A BUN-to-creatinine ratio above 20 often points to dehydration or reduced blood flow to the kidneys rather than structural kidney damage. BUN alone isn’t a reliable indicator of kidney health because it fluctuates with protein intake, hydration, and other non-kidney factors.
Your panel may also include electrolytes like potassium and phosphorus, since damaged kidneys struggle to keep these in balance. These results help your doctor understand how well your kidneys are handling their full range of jobs, not just waste filtering.
CKD Stages and What Your Numbers Mean
If your tests confirm kidney disease, your eGFR places you into one of five stages:
- Stage 1 (eGFR 90+): Normal filtration, but other signs of damage like protein in the urine are present.
- Stage 2 (eGFR 60–89): Mildly reduced function with evidence of kidney damage.
- Stage 3a (eGFR 45–59): Mild to moderate loss of function.
- Stage 3b (eGFR 30–44): Moderate to severe loss of function.
- Stage 4 (eGFR 15–29): Severe loss of function.
- Stage 5 (eGFR below 15): Kidney failure.
An important detail: an eGFR between 60 and 89 with no protein in the urine and no other signs of damage does not qualify as CKD. Mild filtration dips can be normal, especially with aging. The diagnosis requires either an eGFR below 60 or clear evidence of damage, and the abnormality has to last at least three months to rule out temporary causes.
Imaging Tests
When blood and urine tests suggest a problem, your doctor may order an ultrasound of your kidneys. This painless scan uses sound waves to create a picture of your kidneys and can reveal kidney stones, cysts, tumors, blockages, unusual size or shape, and fluid buildup (called hydronephrosis). It also shows how blood flows to and within your kidneys. A dark spot on the image could indicate a cyst or abscess that needs further evaluation.
A CT scan provides more detailed images and is sometimes used when the ultrasound findings are unclear, or when your doctor needs a closer look at stones, masses, or structural abnormalities. Neither test involves any kidney-specific preparation beyond what your imaging center instructs.
Kidney Biopsy
A biopsy is reserved for situations where blood tests, urine tests, and imaging haven’t provided a clear diagnosis, or when your doctor needs to determine the exact type of kidney disease to guide treatment. Common reasons include unexplained blood in the urine originating from the kidneys, excessive or rising protein levels, or kidney function declining faster than expected.
During the procedure, you lie on your stomach or side while a doctor uses ultrasound to guide a thin needle through your skin and into your kidney to collect a small tissue sample. You’re awake but receive local numbing medication and sometimes mild sedation through an IV. The tissue goes to a lab where it’s examined under a microscope. Biopsies are also used to check on transplanted kidneys that aren’t functioning well.
How to Prepare for Kidney Testing
A renal function panel typically requires fasting for 8 to 12 hours beforehand. During that time, you can drink plain water but should avoid food, gum, smoking, and exercise. Let your doctor know about any vitamins, supplements, or over-the-counter medications you take, since some can affect creatinine levels or other results. Don’t stop prescription medications unless your doctor specifically tells you to.
For the urine test, no fasting is needed. A first-morning sample is ideal because it’s the most concentrated, but your doctor will tell you when and how to collect it.
How Often You Should Be Tested
If you have diabetes or high blood pressure, the two leading causes of kidney disease, guidelines recommend kidney function testing at least once a year when your eGFR is 60 or above. If your eGFR drops into the 30 to 59 range, testing frequency increases to every 3 to 6 months. General risk factor reassessment, including blood pressure, blood sugar control, and kidney markers, is recommended every 3 to 6 months for people with diabetes and existing kidney concerns.
If you don’t have known risk factors and your initial screening comes back normal, routine annual testing usually isn’t necessary. But if you have a family history of kidney disease, are over 60, or have a history of heart disease, asking your doctor about periodic screening is reasonable.