Kidney function is checked primarily through two simple tests: a blood test that estimates how well your kidneys filter waste, and a urine test that detects protein your kidneys shouldn’t be letting through. Most people get both done at the same time, and results are usually available within a day or two. Together, these tests can catch kidney problems years before symptoms appear.
The Two Core Tests
The blood test measures creatinine, a waste product your muscles produce at a fairly steady rate. Your kidneys filter it out, so when they’re not working well, creatinine builds up in your blood. Rather than just reporting the raw creatinine number, labs plug it into a formula that estimates your glomerular filtration rate, or eGFR. This number tells you what percentage of normal filtering capacity your kidneys still have. An eGFR of 90 or above is considered normal. Between 60 and 89 may indicate mild loss. Below 60 sustained over three months is the threshold for chronic kidney disease, and below 15 means the kidneys are close to failure.
The urine test measures how much albumin (a blood protein) is leaking into your urine and reports it as a ratio called UACR. Healthy kidneys keep albumin in the blood, so finding it in urine is an early warning sign. A UACR below 30 mg/g is normal. Between 30 and 299 mg/g is considered moderately increased and signals higher risk for kidney failure and cardiovascular events like heart attack or stroke. At 300 mg/g or above, that risk becomes significantly higher. Importantly, a confirmed elevated UACR twice within six months can mean kidney disease even if your eGFR looks fine above 60.
What Your Blood Results Actually Mean
Your lab report will likely include a few kidney-related values beyond eGFR. Blood urea nitrogen, or BUN, is another waste marker with a normal range of 5 to 20 mg/dL. But BUN is far less reliable on its own because it swings with things that have nothing to do with your kidneys. A high-protein diet, dehydration, fever, infection, or even gastrointestinal bleeding can push BUN up. A low-protein diet or liver problems can push it down. Doctors look at BUN alongside creatinine rather than relying on it alone.
One important change in recent years: the formula used to calculate eGFR no longer includes a race variable. In 2021, the National Kidney Foundation and the American Society of Nephrology recommended immediate adoption of the updated CKD-EPI 2021 equation, which removed race from the calculation. The older formula had adjusted results based on whether a patient was identified as Black, which delayed diagnoses and referrals for many people. The current consensus is that race is a social construct, not a biological variable, and has no place in kidney function assessment.
Preparing for Kidney Tests
Your doctor may ask you to fast overnight before a blood draw, though this varies by lab. You may also be told to avoid eating meat for a set period beforehand, since meat temporarily raises creatinine levels. If you take a creatine supplement for exercise or muscle building, you’ll likely need to stop before testing because creatine directly affects creatinine measurements.
One thing to be aware of: drinking large amounts of water right before a blood test can temporarily lower your creatinine reading. This only masks the result for a short time. It does not improve kidney function or reverse any damage, and it can lead to a falsely reassuring number that delays a real diagnosis.
When a Cystatin C Test Is More Accurate
Creatinine-based eGFR has a significant blind spot. Because creatinine comes from muscle, people with unusually high or low muscle mass can get misleading results. Research has found that creatinine-based formulas misclassify kidney disease roughly 50% of the time in certain populations. Cystatin C, a different protein produced by nearly all cells in the body, is much less affected by gender, age, body size, or nutritional status. Formulas based on cystatin C still misclassify about 30% of the time, but that’s a meaningful improvement.
Guidelines recommend cystatin C testing for adults whose creatinine-based eGFR falls between 45 and 60 but who don’t show other signs of kidney damage, as a way to confirm whether kidney disease is truly present. It’s also considered a superior marker for people with conditions that affect muscle mass or creatinine processing, including sickle cell disease, muscular dystrophy, spina bifida, and those undergoing chemotherapy. Cystatin C picks up earlier, more subtle changes in kidney function that creatinine misses.
How Often to Get Tested
If you have diabetes, hypertension, or are over 50, you fall into a higher-risk group where annual kidney screening is strongly recommended. For people with type 2 diabetes, screening should begin at the time of diagnosis. For type 1 diabetes, it typically starts five years after diagnosis, since type 1 tends to be caught quickly while type 2 often goes undetected for years before it’s identified.
Once kidney disease is detected, how often you’re monitored depends on how advanced it is. Early-stage disease with a normal eGFR and only mildly elevated protein warrants testing about once a year. Moderate disease bumps that to twice a year. More advanced stages call for testing three or four times annually. Your doctor will adjust this schedule based on your specific numbers and how quickly they’re changing.
Imaging Tests for Kidney Problems
Blood and urine tests measure how well your kidneys work, but they can’t show you what the kidneys look like. When a structural problem is suspected, such as kidney stones, cysts, or blockages, imaging comes next. Ultrasound is typically the first choice because it uses no radiation, costs less, and is widely accessible. It works well for initial evaluation, and it’s preferred for younger patients, pregnant women, or anyone who needs repeated monitoring without radiation exposure.
CT scans are the more precise option. They offer higher sensitivity and specificity for detecting kidney stones and provide detailed anatomic information that ultrasound can’t match. Most urologists still rely on CT to confirm what they’re dealing with before recommending any procedure. In practice, many people start with an ultrasound, and a CT is ordered only if more detail is needed.
At-Home Kidney Screening
There is at least one FDA-cleared at-home kidney test available: the Minuteful kidney test, made by Healthy.io. It’s a urine test you do at home that measures albumin and creatinine levels in your urine and calculates the ACR. The test uses your smartphone camera to read a standard urine dipstick, and it’s classified as a prescription-use product, meaning your doctor has to order it.
The accuracy is reasonable for a screening tool. In FDA review, the test showed 92.7% exact agreement with a clinical laboratory device for the albumin-creatinine ratio. Per individual measurement, albumin agreement was 92.1% and creatinine was 88.2%. These are semi-quantitative results, meaning they place you in a category (normal, moderately increased, severely increased) rather than giving a precise number. An abnormal result still needs to be confirmed by a standard lab test. The main value of at-home testing is convenience for people who are at risk and need regular screening but face barriers getting to a lab.
What Abnormal Results Lead To
A single abnormal result doesn’t automatically mean kidney disease. Both eGFR and UACR need to be confirmed with repeat testing over three to six months before a diagnosis is made. Temporary factors like dehydration, intense exercise, a recent high-protein meal, or certain medications can throw off a single result.
If repeat testing confirms a problem, your doctor will look at the combination of your eGFR and UACR to determine the stage and severity. Early-stage kidney disease is often managed with blood pressure control, blood sugar management for diabetics, and dietary adjustments. The goal at that point is slowing progression, since lost kidney function generally doesn’t come back. Catching it early through routine screening is what gives you the most options.