Kidney Function Tests — Creatinine, eGFR and BUN
Our kidneys perform several essential jobs. They filter waste products from the blood, regulate fluid and electrolyte balance, help control blood pressure, and produce hormones involved in red blood cell production and bone health.
Kidney function often declines gradually with age, although many older adults maintain good kidney health throughout their lives.
Three commonly ordered blood test results—creatinine, estimated glomerular filtration rate (eGFR), and blood urea nitrogen (BUN)—help doctors evaluate how well the kidneys are working.
Understanding these numbers can be particularly useful for older adults, people with diabetes or high blood pressure, and those taking medications or supplements that may affect kidney function.
Creatinine — What Does It Measure?
Creatinine is a waste product produced mainly from normal muscle metabolism. The kidneys remove it from the bloodstream.
When kidney filtration declines, creatinine often increases.
However, creatinine levels are also influenced by factors unrelated to kidney disease, including:
Muscle mass
Recent strenuous exercise
Eating cooked meat before testing
Certain medications
Creatine supplementation
People with greater muscle mass may have higher creatinine levels despite normal kidney function.
Conversely, older adults with reduced muscle mass may have relatively low creatinine levels even when kidney function is impaired.
This is one reason creatinine should not be interpreted by itself.
eGFR — Estimating Kidney Filtration
Estimated glomerular filtration rate (eGFR) is one of the most useful measures of kidney function.
It estimates how much blood the kidneys filter each minute, adjusted to a standard body surface area.
Most laboratories calculate eGFR using blood creatinine, age, and sex.
The result is expressed in mL/min/1.73 m².
Understanding eGFR Results
90 or higher: Normal or high filtration
60–89: Mildly reduced filtration
45–59: Mild to moderately reduced filtration
30–44: Moderately to severely reduced filtration
15–29: Severely reduced filtration
Below 15: Kidney failure range
These categories require context.
An eGFR between 60 and 89 does not necessarily indicate chronic kidney disease, particularly when there are no other signs of kidney damage.
An eGFR below 60 that persists for at least three months can indicate chronic kidney disease.
A single abnormal result is generally insufficient to establish the diagnosis.
What About Aging?
eGFR tends to decline with age, but this does not mean that every reduction represents dangerous kidney disease.
For example, an otherwise healthy person in their 80s may have an eGFR somewhat below that of a younger adult.
However, reduced kidney filtration should not automatically be dismissed as normal aging.
Doctors consider the rate of change, urine findings, blood pressure, medications, and other medical conditions.
A stable eGFR over several years is generally more reassuring than a rapidly declining value.
BUN — Blood Urea Nitrogen
BUN measures the amount of nitrogen in the blood that comes from urea, a waste product formed when the body metabolizes protein.
The kidneys normally remove urea through urine.
A commonly used BUN reference range is approximately 6–20 mg/dL, although laboratory ranges vary.
BUN can rise when kidney function declines, but it is also influenced by:
Dehydration
High protein intake
Gastrointestinal bleeding
Certain medications
Increased protein breakdown during illness
BUN may be particularly sensitive to changes in hydration.
An elevated BUN does not necessarily indicate kidney disease.
How Does a High-Protein Diet Affect These Tests?
Protein intake can influence kidney-related blood tests, especially BUN.
A high-protein meal or diet can increase urea production, sometimes raising BUN even when kidney function is normal.
Eating cooked meat shortly before a blood test can temporarily increase creatinine.
For people with healthy kidneys, higher protein intake does not automatically mean kidney damage. However, people with established chronic kidney disease may need individualized protein recommendations.
This distinction matters for older adults who are trying to consume enough protein to preserve muscle mass.
What About Creatine Supplements?
Creatine is widely used to improve muscle strength and exercise performance, particularly when combined with resistance training.
It is also being investigated for possible benefits involving brain energy and cognitive health.
Some creatine is converted into creatinine, so supplementation can sometimes increase blood creatinine without indicating actual kidney damage.
This can make creatinine-based eGFR appear lower than the person's true kidney filtration rate.
Research in generally healthy adults has not shown that recommended creatine supplementation routinely causes kidney damage. However, long-term evidence is more limited in people with existing kidney disease.
Anyone taking creatine should tell their healthcare professional before interpreting kidney function tests.
Cystatin C — Another Way to Estimate Kidney Function
Cystatin C is a protein produced by cells throughout the body and removed from the blood through kidney filtration.
Unlike creatinine, cystatin C is much less dependent on muscle mass.
Doctors can use it to calculate another estimate of kidney function.
An equation combining creatinine and cystatin C often provides a more accurate estimate than either marker alone.
This can be especially useful for:
Older adults with low muscle mass
People taking creatine supplements
People with unusually high muscle mass
Patients whose creatinine-based eGFR does not fit their overall health picture
Cystatin C is not perfect. Inflammation, thyroid disorders, steroid medications, and other factors can influence its level.
Nevertheless, it can be a valuable additional test when creatinine results are difficult to interpret.
Don't Forget Urine Testing
Blood tests are only part of evaluating kidney health.
A urine albumin-to-creatinine ratio (uACR) measures whether small amounts of albumin, a blood protein, are leaking into the urine.
Increased urinary albumin can be an early sign of kidney damage, sometimes before eGFR declines.
Common categories are:
Below 30 mg/g: Normal to mildly increased
30–300 mg/g: Moderately increased
Above 300 mg/g: Severely increased
Persistent albumin in the urine is associated with increased risk of kidney disease progression and cardiovascular problems.
For people with diabetes, high blood pressure, or suspected kidney disease, uACR can provide information that blood tests alone may miss.
What Can We Do to Protect Kidney Health?
Several measures can help preserve kidney function:
Maintain healthy blood pressure and blood glucose levels.
Avoid dehydration, particularly during illness involving vomiting or diarrhea.
Review medications that may affect kidney function.
Be cautious with frequent use of NSAID pain relievers such as ibuprofen and naproxen.
Maintain physical activity and muscle strength.
Avoid smoking.
Follow up on persistently abnormal kidney or urine test results.
People with established kidney disease may benefit from medications specifically shown to slow its progression.
The Bottom Line
Creatinine, eGFR, and BUN are useful tests, but none should be interpreted in isolation.
Creatinine can be affected by muscle mass, diet, and creatine supplements. BUN is influenced by protein intake and hydration. eGFR provides an estimate of filtration, not a direct measurement.
For older adults, especially those taking creatine or those with reduced muscle mass, cystatin C and urine albumin testing may provide important additional information.
The most useful approach is to look at trends over time, consider the entire clinical picture, and investigate meaningful changes rather than focusing on a single laboratory number.
Learn More
An illustrated explanation of the major kidney function tests, including BUN, creatinine, eGFR, and urine testing.
Explains how kidney filtration is estimated, what the numbers mean, and why results may vary with age and muscle mass.
Explains why cystatin C can provide a more accurate estimate of kidney function in some older adults and people whose muscle mass makes creatinine results less reliable.
Urine Albumin-to-Creatinine Ratio (uACR) — National Kidney Foundation
Explains how a simple urine test can detect early kidney damage, sometimes before blood tests show declining kidney function.