Kidney Function Tests: A Guide to BUN, Creatinine, and eGFR
The kidneys filter waste products from the blood, balance fluids and electrolytes, and help regulate blood pressure. Because kidney function can decline gradually without obvious symptoms, laboratory tests are often used to estimate how well the kidneys are working. This guide explains the most common kidney function tests, what they measure, and how results are interpreted.
Key takeaways
- Blood urea nitrogen (BUN) and serum creatinine are the two most commonly used blood tests to assess kidney function.
- Estimated glomerular filtration rate (eGFR) is calculated from creatinine and other factors to estimate how well the kidneys filter blood.
- A urine albumin-to-creatinine ratio can detect early kidney damage before eGFR declines.
- Single test results are rarely interpreted alone; trends over time and the full clinical picture matter.
- Reference ranges vary by laboratory, so results should be discussed with a qualified healthcare professional.
What Kidney Function Tests Measure
Kidney function tests evaluate how effectively the kidneys filter waste from the blood and maintain fluid and electrolyte balance. The most frequently ordered blood tests are blood urea nitrogen (BUN) and serum creatinine. BUN measures the amount of nitrogen in the blood that comes from urea, a waste product formed when the body breaks down protein. Creatinine is a waste product from normal muscle metabolism that healthy kidneys filter out of the blood.
Because creatinine production is relatively stable in a given person, an elevated serum creatinine level can indicate reduced kidney filtration. BUN, by contrast, can be influenced by factors beyond kidney function, including hydration status, protein intake, liver function, and gastrointestinal bleeding. For this reason, BUN and creatinine are usually interpreted together rather than in isolation.
Estimated Glomerular Filtration Rate (eGFR)
The estimated glomerular filtration rate (eGFR) is a calculated value that estimates how much blood the kidneys filter per minute. It is derived from serum creatinine along with variables such as age and sex, which affect creatinine production and muscle mass. eGFR is widely used to stage chronic kidney disease and to track changes in kidney function over time.
An eGFR result is an estimate, not a direct measurement of filtration. It may be less accurate in people with unusually high or low muscle mass, in pregnant individuals, or in those with acute changes in kidney function. When a more precise measurement is needed, a clinician may order a timed urine collection or other specialized testing.
Urine Testing for Kidney Health
Blood tests alone do not capture all aspects of kidney health. A urine albumin-to-creatinine ratio (UACR) measures the amount of albumin, a protein, relative to creatinine in a urine sample. Albumin is normally retained in the blood; when it appears in urine, it can be an early sign of kidney damage, particularly in people with diabetes or high blood pressure.
A urinalysis can also detect blood, protein, white blood cells, or other abnormal findings in the urine. Because albuminuria may appear before eGFR declines, urine testing is often recommended alongside blood testing for a more complete assessment of kidney function.
Interpreting Results and When Testing Is Used
Kidney function tests are commonly ordered as part of routine health screening, especially for people with conditions that increase kidney disease risk, such as diabetes, hypertension, or a family history of kidney problems. They may also be ordered to monitor the effects of medications that can affect the kidneys or to evaluate symptoms such as swelling, fatigue, or changes in urination.
Results are interpreted in the context of the individual's medical history, medications, hydration status, and other laboratory findings. A single abnormal value does not necessarily indicate kidney disease, and a normal value does not rule out all kidney conditions. Healthcare professionals typically review trends over time and may repeat testing or order additional evaluations before drawing conclusions.
Frequently Asked Questions
what does bun lab test mean
A BUN lab test measures the amount of urea nitrogen in the blood. Urea is a waste product formed when the body breaks down protein, and it is normally removed by the kidneys. A BUN test is often ordered with a creatinine test to help assess kidney function, though BUN can also be affected by hydration, diet, liver function, and other factors.
what lab test shows kidney function
Several laboratory tests help assess kidney function. Serum creatinine and blood urea nitrogen (BUN) are common blood tests, and estimated glomerular filtration rate (eGFR) is calculated from creatinine and other variables. Urine tests, such as the urine albumin-to-creatinine ratio, can detect early kidney damage. These tests are typically interpreted together.
what is bun on lab test
On a lab test, BUN stands for blood urea nitrogen. It reflects the amount of nitrogen in the blood that comes from urea, a waste product of protein metabolism. The kidneys filter urea from the blood, so BUN levels can provide information about kidney function, although many non-kidney factors can also influence the result.
can kidney function tests be normal with kidney disease
It is possible for some kidney function tests to appear normal in the early stages of kidney disease. For example, eGFR may remain within the reference range while urine albumin is already elevated. Because different tests detect different aspects of kidney health, clinicians often use blood and urine testing together and monitor results over time.
Sources
Ready to get tested? Compare markers, check any fasting guidance, then secure the panel via the BloodTestPanels partner at your convenience.
Order BUN/Creatinine Ratio TestReviewed by the BloodTestPanels Editorial Team
The BloodTestPanels editorial team reviews laboratory panels against published reference material before publication.