Your kidneys filter about half a cup of blood every minute, removing waste and extra fluid as urine. Checking how well they work usually starts with a simple blood test called a serum creatinine test, which is used to calculate an estimated glomerular filtration rate (eGFR). A urine test, most often a urine albumin-to-creatinine ratio, checks for protein leaking into urine. Imaging tests such as ultrasound, CT, or MRI are used when a structural problem is suspected, not as routine screening.
How To Check Kidney Function Blood Urine And Imaging: The Three Main Approaches
Doctors rarely rely on a single test. They combine blood, urine, and sometimes imaging to build a full picture of kidney health.
Blood tests measure how well the kidneys clear waste. Urine tests detect damage that blood tests can miss early on. Imaging shows the physical structure — size, shape, blockages, cysts, or stones. Each answers a different question.
A person with early kidney disease may have a normal eGFR but abnormal protein in the urine. Someone else may have a low eGFR from a structural cause that only imaging can reveal. This is why the three approaches work together rather than as substitutes.
What Does a Blood Test for Kidney Function Measure?
The main blood test measures creatinine, a waste product from normal muscle activity that healthy kidneys filter out. Because creatinine production stays fairly steady, its blood level reflects how well the kidneys are clearing it.
Creatinine alone is hard to interpret, so labs plug it into a formula along with age and sex to produce an estimated glomerular filtration rate (eGFR). This number estimates how much blood your kidneys filter per minute, adjusted for body size. A normal eGFR is generally considered to be 90 mL/min/1.73 m² or higher in adults.
Doctors often stage kidney function using eGFR ranges. An eGFR below 60 for three months or longer generally indicates chronic kidney disease, regardless of cause. An eGFR below 15 is described as kidney failure. These are established thresholds used in clinical practice, but a single low reading does not confirm chronic disease — the pattern over time matters.
Blood urea nitrogen (BUN) is sometimes checked alongside creatinine. BUN can rise with kidney problems, but it is also affected by dehydration, diet, liver disease, and other factors, so it is less specific on its own.
What Can a Urine Test Reveal About Kidney Health?
A urine test can detect damage before eGFR drops. The most useful measure is albumin, a protein that should mostly stay in the blood. When kidney filters are damaged, small amounts leak into urine.
The standard test is the urine albumin-to-creatinine ratio (UACR), usually done on a single urine sample. A UACR below 30 mg/g is considered normal. A result between 30 and 300 mg/g is called moderately increased albuminuria, and above 300 mg/g is severely increased. These categories come from widely used clinical guidelines and help predict kidney and heart risk.
A urinalysis also checks for blood, white blood cells, and other substances. Blood in urine can point to infection, stones, or kidney inflammation. A dipstick test is quick but less precise than a lab-measured UACR.
One practical point people often miss: a single abnormal urine result is not a diagnosis. Temporary rises in urine protein can happen with fever, intense exercise, or urinary tract infection. Doctors usually repeat the test to confirm a pattern.
When Are Imaging Tests Used to Check Kidneys?
Imaging is not part of routine kidney screening. It is used when blood or urine results suggest a problem, or when symptoms point to a structural cause.
Ultrasound is usually the first imaging choice. It uses sound waves, involves no radiation, and shows kidney size, shape, fluid buildup, and blockages. It is safe in pregnancy.
CT scans give detailed cross-sectional images and are often used to look for stones, tumors, or injuries. Some CT scans use iodine-based contrast dye, which can stress kidneys that are already weak. Doctors weigh this risk against the value of the information.
MRI uses magnetic fields and can show soft tissue and blood vessels in detail. A specific type called magnetic resonance angiography can map the arteries feeding the kidneys. Some MRI contrast agents carry their own risks in people with advanced kidney disease, so this is factored into the decision.
Other options include a nuclear medicine scan, which measures how well each kidney filters, and a kidney biopsy, which removes a tiny tissue sample. A biopsy is not imaging, but it is sometimes needed when the cause of kidney disease is unclear.
How Do Doctors Decide Which Tests to Order?
The choice depends on why testing is being done in the first place.
- Routine screening: blood creatinine with eGFR and a urine albumin test
- Monitoring known kidney disease: repeat blood and urine tests at set intervals
- Suspected stones or blockage: ultrasound or CT
- Blood in urine or suspected tumor: CT or ultrasound
- Unclear cause of kidney failure: sometimes a biopsy
People with diabetes or high blood pressure are commonly screened because both conditions are leading causes of kidney disease. A family history of kidney failure, certain medications, and some autoimmune conditions also raise the priority for testing.
There is no single test that covers everything. A normal eGFR does not rule out early damage, and a clean ultrasound does not rule out a filtering problem. The value comes from combining results and tracking them over time.
Understanding Your Results and What They Mean
Numbers matter, but context matters more. A result that is slightly outside the normal range may be meaningless on its own, while a result inside the range may still signal a problem when paired with other findings.
Age affects eGFR. A gradual decline in filtering rate is common with age and does not always mean disease. Muscle mass also matters — people with low muscle mass may have misleadingly low creatinine levels, while muscular people may have higher levels without any kidney problem.
Trends over months and years are usually more informative than any single test. A slow, steady decline is different from a sudden drop. A stable low number may be less concerning than a number that keeps falling.
Anyone with abnormal results should discuss them with a clinician who can interpret them alongside symptoms, medical history, and other tests. Online calculators and at-home kits can offer rough guidance, but they cannot replace a proper evaluation.
What About Home Testing Kits?
At-home urine test strips are widely sold and can detect protein or blood in urine. They are not a substitute for a lab-based urine albumin-to-creatinine ratio.
Home strips can produce false positives and false negatives. A positive result should prompt a follow-up with a clinician, not a diagnosis. There are no reliable at-home tests for eGFR, because that requires a blood draw and a lab calculation.
Some companies now offer mail-in blood test kits that measure creatinine. The accuracy of these kits varies, and results still need interpretation by a clinician familiar with the person’s history.
Frequently Asked Questions
What is the first test usually done to check kidney function?
A blood test for creatinine, used to calculate eGFR, is typically the first step. A urine albumin test is often added at the same time.
Can a blood test alone tell if my kidneys are healthy?
No. A blood test can miss early kidney damage that shows up first as protein in the urine, so urine testing is usually needed too.
Do I need imaging if my blood and urine tests are normal?
Usually not. Imaging is reserved for cases where a structural problem is suspected based on symptoms, abnormal lab results, or specific risk factors.
How often should kidney function be checked?
Frequency depends on your risk factors and any existing kidney disease. People with diabetes, high blood pressure, or a prior abnormal result are often tested at least once a year, but your clinician sets the schedule.
