Kidney Function Testing: What Your Lab Results Mean

Key takeaways

• BUN and creatinine are the two most common blood tests used to assess kidney function.
• eGFR is calculated from creatinine and other factors to estimate how well the kidneys filter blood.
• Normal reference ranges vary by laboratory, age, sex, and other factors, so results should be interpreted in context.
• A single abnormal result does not necessarily mean kidney disease; trends over time and additional testing are often needed.
• Urine tests such as urinalysis and albumin-to-creatinine ratio can detect problems that blood tests may miss.

What Kidney Function Tests Measure

Kidney function tests evaluate how effectively the kidneys filter waste products and balance fluids and electrolytes. 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 activity that healthy kidneys filter out of the blood.

Because creatinine production is relatively stable, an elevated serum creatinine level can suggest reduced kidney filtration. However, creatinine can be affected by muscle mass, diet, and certain medications. BUN can be influenced by hydration, protein intake, liver function, and gastrointestinal bleeding. For these reasons, BUN and creatinine are usually interpreted together rather than in isolation.

Estimated Glomerular Filtration Rate (eGFR)

The estimated glomerular filtration rate (eGFR) is a calculation based on serum creatinine along with variables such as age and sex. It estimates how much blood the kidneys filter per minute and is often reported alongside creatinine results. Many laboratories automatically calculate eGFR whenever a creatinine test is ordered.

eGFR is commonly used to stage chronic kidney disease, with lower values indicating reduced filtration. However, eGFR is an estimate and 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. Clinicians may order additional tests, such as a 24-hour urine collection or cystatin C, when a more precise assessment is needed.

Urine Tests and Other Kidney Assessments

Blood tests alone do not provide a complete picture of kidney health. A urinalysis can detect blood, protein, glucose, or cells in the urine that may indicate kidney damage or another condition. The presence of protein in the urine, called proteinuria, is an important early sign of kidney disease, particularly in people with diabetes or high blood pressure.

The urine albumin-to-creatinine ratio (UACR) is a more specific test that measures how much albumin, a type of protein, is excreted relative to creatinine. A persistently elevated UACR can indicate kidney damage even when eGFR is normal. Imaging studies such as ultrasound and, in some cases, a kidney biopsy may also be used when the cause of abnormal kidney function is unclear.

Interpreting Results and When Testing Is Used

Reference ranges for BUN, creatinine, and eGFR differ among laboratories and depend on factors such as age, sex, and muscle mass. A result slightly outside the reference range may not be clinically significant, while a result within range does not rule out kidney disease. Healthcare providers interpret results in the context of symptoms, medical history, medications, and other laboratory findings.

Kidney function testing may be ordered as part of routine health screening, to monitor chronic conditions such as diabetes or hypertension, to evaluate symptoms such as swelling or fatigue, or to monitor the effects of medications that can affect the kidneys. Because kidney function can change over time, serial measurements are often more informative than a single test. Anyone with concerns about kidney function should discuss results with a qualified healthcare professional.

Frequently Asked Questions

Sources

• National Institute of Diabetes and Digestive and Kidney Diseases: Kidney Tests
• MedlinePlus: BUN (Blood Urea Nitrogen) Test
• MedlinePlus: Creatinine Test
• Centers for Disease Control and Prevention: Chronic Kidney Disease Basics
• Mayo Clinic: Chronic Kidney Disease - Diagnosis and Treatment