Kidney Function Testing: Creatinine, BUN, and eGFR

Kidney function testing uses a small set of blood and urine tests to estimate how well your kidneys filter waste and whether protein is leaking into your urine. The most common combination pairs creatinine, blood urea nitrogen (BUN), and an estimated glomerular filtration rate (eGFR). Read together, these markers help clinicians screen for chronic kidney disease, monitor an existing diagnosis, and check whether medications are stressing the kidneys.

What kidney function testing measures

Each kidney contains about a million tiny filters called nephrons. They clear waste and extra fluid from the blood while holding on to the substances your body needs. Because the kidneys can lose a large share of their capacity before symptoms appear, laboratory testing often catches decline years before a person notices anything wrong.

A basic metabolic panel or a dedicated renal function panel usually includes creatinine, BUN, electrolytes, calcium, and glucose. A clinician may add a urine albumin test or a complete urinalysis. No single result stands alone. Doctors read the numbers as a pattern, then factor in age, blood pressure, medications, and your medical history.

Creatinine: the core filtration marker

Creatinine is a waste product created when muscles use energy. Healthy kidneys remove most of it from the blood with every pass, so the blood level stays fairly steady. When filtration slows, creatinine rises. That makes it the primary ingredient in most kidney estimates.

Creatinine also depends on muscle mass. A muscular young adult can have a higher baseline than a smaller or older person and still have normal kidney function. Reference ranges differ between laboratories, so always compare your value to the range printed on your own report instead of a number you found online.

BUN and the BUN-to-creatinine ratio

Blood urea nitrogen measures urea, a waste product formed when the body breaks down protein. The liver makes urea, and the kidneys clear it. A high BUN can point to reduced kidney filtration, but it can also reflect dehydration, a high-protein diet, bleeding in the digestive tract, or a catabolic state. A low BUN may simply mean a low-protein diet or liver disease.

Because BUN and creatinine move for different reasons, clinicians often look at the BUN-to-creatinine ratio. A typical ratio sits around ten to one to twenty to one. A ratio well above that range can suggest dehydration or reduced blood flow to the kidneys, while a normal or low ratio with both values high points more toward intrinsic kidney damage. The ratio is a clue, not a diagnosis.

eGFR: turning creatinine into a filtration score

The estimated glomerular filtration rate converts a creatinine result into a number that approximates how much blood your kidneys filter each minute. Modern equations factor in age and sex, and some older formulas also considered race. Professional groups now recommend equations that do not adjust for race, because race is a social category rather than a biological reason for differences in kidney function.

Results are grouped into categories. An eGFR of 90 or above with no other signs is generally considered normal. Values between 60 and 89 suggest mildly reduced filtration. A result below 60 that persists for three months or longer supports a chronic kidney disease diagnosis, and lower categories mark more advanced stages. A single low eGFR does not prove chronic disease, because acute illness, dehydration, and some medicines can drop the number temporarily.

Urine tests: albumin, urinalysis, and the ACR

Blood tests reveal filtration. Urine tests reveal damage. The urine albumin-to-creatinine ratio compares the amount of albumin, a blood protein, to creatinine in a urine sample. Healthy kidneys keep albumin in the blood. When the filters leak, albumin appears in urine. Higher ratios indicate more leakage and a greater risk of kidney and heart problems.

A standard urinalysis adds information on blood, white cells, glucose, and casts. Your clinician may combine eGFR stages with albumin categories to estimate overall risk. This two-part view is why kidney testing rarely ends with a single blood draw.

How to read your results together

Imagine two people with the same low eGFR. One has heavy albumin in the urine and rising creatinine. The other has a stable low number, no protein, and no symptoms. Their outlook and management differ. This is why clinicians rarely act on one value. They watch trends across time, note how quickly numbers change, and interpret each result against the rest of the panel.

Bring all your past results to appointments when you can. A trend line answers questions that a single snapshot cannot. If a value moved sharply, your clinician may repeat the test before drawing conclusions.

What can raise or lower your numbers

Many everyday factors shift kidney markers without any disease. Intense exercise, a very high-protein meal, creatine supplements, and dehydration can push creatinine or BUN upward. Certain medications, including some pain relievers, blood pressure drugs, and antibiotics, affect results as well. Pregnancy, muscle loss, and severe liver disease can lower creatinine and make filtration look better than it is. Tell your provider about every supplement and prescription you take so results are read in context.

Who should get kidney function testing

Screening is most valuable for people with higher risk. That group includes anyone with diabetes, high blood pressure, heart disease, obesity, or a family history of kidney failure. Age also matters, since filtration naturally declines over time. A clinician may recommend yearly testing for these groups and more frequent monitoring if results are already abnormal. People with no risk factors may still receive testing as part of routine blood work.

Preparation and when to talk to a provider

Most kidney panels need no special preparation, but a few steps improve accuracy. Drink normal amounts of water unless your provider tells you otherwise, since dehydration can distort results. Avoid intense exercise for a day before the draw, and ask whether any of your medications or supplements should be paused. If a fasting glucose or lipid test is bundled with the kidney panel, you may need to fast for eight to twelve hours.

Reach out to a healthcare provider promptly if you notice swelling in your legs or face, a lasting change in urination, blood in the urine, persistent fatigue, or new shortness of breath. These signs deserve assessment rather than a wait-and-see approach. This guide is educational and does not replace advice from a qualified clinician who knows your history.