Learn About Kidney Function Tests and Results

What Kidney Function Tests Measure and Why Doctors Order Them

Kidney function tests are blood and urine tests that show how well your kidneys are working. Your kidneys filter waste products from your blood and help control water balance, blood pressure, and red blood cell production. When kidneys don't work properly, waste builds up in your body and can cause serious health problems. Doctors order these tests to catch kidney problems early, monitor existing kidney disease, or check if medications are affecting your kidney health.

The most common kidney function tests measure substances in your blood and urine. These tests give doctors a picture of how efficiently your kidneys filter waste. According to the National Kidney Foundation, about 37 million Americans have chronic kidney disease, but many don't know it because early stages often have no symptoms. Kidney disease is the ninth leading cause of death in the United States.

Doctors may order kidney function tests during routine check-ups, before starting certain medications, or if you have symptoms like fatigue, swelling in the legs or face, or changes in urination patterns. People with diabetes, high blood pressure, or a family history of kidney disease should have these tests regularly. Your doctor might also order them if you're taking medications that can affect kidney function, such as certain blood pressure medicines or pain relievers.

Understanding what these tests measure helps you work with your doctor to maintain kidney health. The results show whether your kidneys are filtering properly and whether waste is building up in your system. Early detection through these tests gives you time to make lifestyle changes or start treatment before kidney damage becomes severe.

Practical Takeaway: If your doctor recommends kidney function tests, ask why they're being ordered and what specific concerns they're checking for. This helps you understand your health status and take steps to protect your kidney health.

Creatinine Tests: Understanding Your Results

Creatinine is a waste product that your muscles produce constantly. Your kidneys filter creatinine from your blood, and it leaves your body through urine. A creatinine blood test measures how much creatinine is in your blood. If your kidneys aren't filtering well, creatinine builds up. The amount of creatinine in your blood reflects how well your kidneys are working.

Normal creatinine levels in blood typically range from 0.7 to 1.3 milligrams per deciliter (mg/dL), though this varies between labs and between men and women. Men usually have slightly higher creatinine levels than women because they typically have more muscle mass. Age matters too—older adults may have lower creatinine levels as muscle mass naturally decreases with age. A single high creatinine result doesn't always mean kidney disease; your doctor considers your age, sex, weight, and muscle mass when interpreting results.

Creatinine levels can be affected by several factors beyond kidney function. Medications, dehydration, intense exercise, or a high-meat diet can temporarily raise creatinine levels. Some medications like certain antibiotics or blood pressure drugs can also affect these numbers. This is why doctors typically look at creatinine trends over time rather than relying on a single test result.

The glomerular filtration rate (GFR) is calculated using your creatinine level, age, sex, and race. The GFR shows how many milliliters of waste your kidneys filter each minute. A GFR of 60 or higher is considered normal kidney function. GFR between 30 and 59 indicates mild to moderate kidney disease. A GFR below 30 suggests severe kidney disease. Your doctor uses the GFR to stage your kidney disease and determine what treatment or monitoring you need.

Practical Takeaway: When you receive creatinine test results, ask your doctor to explain your specific numbers and what they mean for your kidney health. Request your GFR calculation if it wasn't provided, as this gives a clearer picture of kidney function than creatinine alone.

Blood Urea Nitrogen (BUN) and Its Meaning

Blood urea nitrogen (BUN) is another waste product that your body produces when protein breaks down. Like creatinine, your kidneys filter BUN from your blood, and it leaves through urine. A BUN test measures how much of this waste remains in your blood. If your kidneys aren't filtering properly, BUN levels rise. Normal BUN levels typically range from 7 to 20 milligrams per deciliter (mg/dL), though different labs may have slightly different ranges.

BUN is more sensitive to changes in your body than creatinine. Your diet, hydration level, liver function, and protein intake all affect BUN levels. Someone who eats a lot of protein may have higher BUN even with normal kidney function. Dehydration raises BUN because there's less fluid to dilute the waste products. This is why doctors don't rely solely on BUN results but look at the relationship between BUN and creatinine together. The BUN-to-creatinine ratio helps doctors determine whether high levels reflect kidney problems or other factors like dehydration or diet.

A high BUN-to-creatinine ratio (typically above 20:1) may suggest dehydration, bleeding, or protein breakdown from illness or starvation. A low ratio might indicate liver disease or malnutrition. When both BUN and creatinine are high together, it more strongly suggests kidney problems. Your doctor interprets these results in the context of your overall health, symptoms, and medical history.

Certain medications affect BUN levels. Diuretics, antibiotics, and corticosteroids can raise BUN. Some medications lower it. If you're taking multiple medications, mention this when your doctor reviews your kidney function test results. Changes in BUN over time matter more than a single number, so your doctor may recommend repeat testing to track trends.

Practical Takeaway: Don't be alarmed by a single high BUN result. Ask your doctor to check your hydration status and dietary protein intake, then discuss whether repeat testing or dietary changes are recommended.

Glomerular Filtration Rate (GFR) and Kidney Disease Stages

The glomerular filtration rate (GFR) measures how many milliliters of blood your kidneys filter each minute. This is considered the best measure of overall kidney function. Your GFR is calculated using your creatinine level along with your age, sex, and race. Some newer calculations don't use race as a factor, as research has shown that including race can mask kidney disease in Black patients. Ask your doctor which calculation method they're using.

Doctors use GFR numbers to place patients into five stages of chronic kidney disease (CKD). Stage 1 (GFR 90 or higher) means normal kidney function, though there may be signs of kidney damage. Stage 2 (GFR 60-89) represents mild loss of kidney function. Stage 3a (GFR 45-59) and 3b (GFR 30-44) indicate moderate kidney function loss. Stage 4 (GFR 15-29) means severe kidney function loss, and Stage 5 (GFR below 15) is kidney failure requiring dialysis or transplant. According to the CDC, about 1 in 7 American adults has chronic kidney disease, with many in earlier stages that can be managed.

Your GFR helps your doctor decide what monitoring or treatment you need. If you have Stage 3 or higher CKD, your doctor may refer you to a nephrologist (kidney specialist). People in earlier stages can often slow kidney disease progression through controlling blood pressure, managing blood sugar if diabetic, and making lifestyle changes. Your doctor may prescribe specific medications that protect kidney function even if you don't have high blood pressure.

GFR can change over time. Some people's kidney function stabilizes or even improves slightly with proper management. Others experience gradual decline. Regular testing helps track your GFR trend, which is more meaningful than any single result. If your GFR drops quickly, your doctor may order additional tests or imaging to find the cause.

Practical Takeaway: Learn your GFR number and which CKD stage it puts you in. Ask your doctor what this means for your health going forward and what steps you should take to protect your remaining kidney function.

Electrolytes and Other Important Kidney Function Tests

Beyond creatinine and BUN, kidney function tests include