Kidney function markers in blood tests are an important group of indicators used to assess the kidneys’ ability to filter blood and remove waste products from the body. Among them, Creatinine, Urea or BUN, and eGFR are commonly used to monitor kidney function. When these markers become abnormal, doctors may suspect impaired kidney function, dehydration, or other health conditions.
However, kidney health should not be evaluated based on a single test result alone. Creatinine levels can be affected by muscle mass, diet, and other factors, while Urea may also vary depending on protein intake or hydration status. Therefore, blood test results are usually interpreted together with eGFR, urine tests, and the individual’s overall health condition. According to the NIDDK, two important groups of tests for detecting and monitoring chronic kidney disease are the assessment of glomerular filtration rate and the measurement of albumin in the urine.
What Is the Function of the Kidneys?
The kidneys are vital organs that filter the blood and remove waste products and excess water from the body through urine. In addition to filtration, the kidneys also help maintain electrolyte balance, regulate blood pressure, and support many other physiological processes.
When kidney function declines, waste products such as creatinine and urea may accumulate in the blood. This is why kidney-related blood tests are widely used in routine health screenings and in monitoring people who are at risk of kidney disease.
Chronic kidney disease can progress silently over a long period without causing obvious symptoms. Therefore, regular testing is particularly important for people with diabetes, high blood pressure, cardiovascular disease, or a family history of kidney disease.
What Kidney Function Markers Are Included in Blood Tests?
1. Creatinine
Creatinine is a waste product produced through normal muscle activity. It enters the bloodstream and is filtered by the kidneys before being eliminated in the urine. Therefore, blood creatinine levels are commonly used to assess the kidneys’ filtering ability.
When kidney filtration declines, creatinine levels tend to rise in the blood. However, creatinine does not reflect kidney function alone because it can also be influenced by muscle mass, age, diet, and other factors.
A person with greater muscle mass may have higher creatinine levels than someone with less muscle mass even if their kidney function is similar. The NIDDK also notes that relying on creatinine alone may provide an incomplete picture of kidney function, particularly in people with unusually high or low muscle mass.
For this reason, doctors usually interpret creatinine together with eGFR for a more accurate assessment.
2. eGFR – Estimated Glomerular Filtration Rate
eGFR, short for estimated Glomerular Filtration Rate, is an estimate of how much blood the kidneys can filter over a certain period of time. It is one of the most important indicators used to assess kidney function.
eGFR is usually calculated based on blood creatinine levels together with information such as age and sex. In some cases, cystatin C may also be used to improve the accuracy of the estimate.
In general, the lower the eGFR, the more reduced the kidneys’ filtering capacity may be. According to the National Kidney Foundation, an eGFR below 60 may indicate abnormal kidney function if it persists, while an eGFR below 15 is often associated with severe kidney failure.
However, a single low eGFR result is not enough to diagnose chronic kidney disease. According to CKD guidelines, an eGFR below 60 that persists for at least 3 months, or persistent evidence of kidney damage, is generally required for the diagnosis of chronic kidney disease.
3. Urea or BUN
Urea is a waste product formed when the body breaks down protein. It is carried in the blood and eliminated mainly through the kidneys.
In some laboratory systems, the result may be reported as BUN, or Blood Urea Nitrogen. This measurement reflects the amount of nitrogen contained in urea.
When kidney function declines, Urea or BUN levels may increase because the kidneys are unable to remove waste efficiently. However, BUN may also rise due to dehydration, a high-protein diet, or other conditions. Therefore, the National Kidney Foundation notes that BUN should generally not be interpreted on its own and should instead be assessed together with creatinine and eGFR.
4. Cystatin C
Cystatin C is a protein produced by many cells in the body and filtered from the blood by the kidneys. A cystatin C test may be used in certain situations to estimate kidney function.
Compared with creatinine, cystatin C is less dependent on muscle mass. Therefore, in people with very high or very low muscle mass, or when creatinine may not accurately reflect kidney filtration, doctors may consider using cystatin C.
The NIDDK states that combining creatinine and cystatin C may provide a more accurate eGFR estimate than using either marker alone in some cases.
Is a High Creatinine Level Dangerous?
A high creatinine level may indicate that the kidneys’ filtering function has declined, but not every case of elevated creatinine means kidney failure.
Several factors can increase creatinine levels, including dehydration, greater muscle mass, strenuous exercise, a diet high in meat, or certain medications. Therefore, doctors need to evaluate creatinine together with eGFR and other clinical information.
If creatinine remains elevated, eGFR decreases, or urine abnormalities are also present, further testing may be needed to identify the underlying cause.
Does a Low eGFR Mean Kidney Failure?
Not necessarily.
A low eGFR may occur temporarily in certain situations, such as dehydration or acute illness. Age can also affect eGFR.
To diagnose chronic kidney disease, doctors usually assess whether the eGFR has remained reduced for 3 months or longer and also look for evidence of kidney damage, such as albumin in the urine.
A single low eGFR result should therefore be monitored and reassessed rather than being interpreted immediately as kidney failure.
Why Is a Urine Test Needed Alongside a Blood Test?
Blood tests help assess the kidneys’ filtering ability, but they may not always detect kidney damage at an early stage.
Albumin in the urine is another important marker. When the kidneys’ filtering units are damaged, the protein albumin may leak into the urine.
The NIDDK considers eGFR and urine albumin to be two key markers in the assessment of chronic kidney disease. Therefore, a comprehensive evaluation of kidney function often includes both blood and urine tests.
When Should Kidney Function Markers Be Checked?
You should consider regular kidney function testing if you belong to a high-risk group.
Groups that commonly require monitoring include people with diabetes, high blood pressure, cardiovascular disease, older adults, and those with a family history of kidney disease.
In addition, symptoms such as swelling in the legs, reduced urine output, unusually frequent urination, persistent foamy urine, fatigue, nausea, or difficulty controlling blood pressure may prompt a doctor to recommend testing.
However, early-stage kidney disease often does not cause obvious symptoms. Therefore, people with risk factors should not wait until symptoms appear before getting tested.
Do Kidney Function Tests Require Fasting?
A standalone creatinine or eGFR test does not usually require fasting. However, if kidney function tests are performed together with blood glucose, lipid profile, or other biochemical tests, the laboratory may recommend fasting before the blood draw.
People undergoing testing should drink water as instructed and avoid becoming excessively dehydrated, as dehydration may affect certain test results.
If you are taking medications or dietary supplements, inform your doctor or healthcare provider. You should not stop taking medication on your own solely for the purpose of having a blood test.
What Factors Can Affect Kidney Function Test Results?
Several factors other than kidney disease can affect creatinine and eGFR results.
Muscle mass is an important factor. People with greater muscle mass may have higher creatinine levels, while older adults or people with low muscle mass may have lower creatinine levels even when kidney function has already declined.
Diet, particularly consuming a large amount of meat before testing, may also affect creatinine levels. Dehydration, acute illness, and certain medications can also alter test results.
The NIDDK notes that eGFR may be less accurate in people with extremely high or low muscle mass or body size, rapidly changing creatinine levels, or unusual dietary patterns.
What Should You Do If Kidney Function Markers Are Abnormal?
If creatinine is elevated, eGFR is reduced, or urea levels are high, you should not self-treat or take kidney supplements without medical advice.
Depending on the situation, a doctor may recommend repeating the blood test to confirm the result, performing a urine test, measuring urine albumin, or carrying out a kidney ultrasound.
More importantly, the underlying cause of the abnormal result needs to be identified. Diabetes and high blood pressure are major risk factors for chronic kidney disease, so controlling blood glucose and blood pressure plays an important role in protecting kidney function.
People should also avoid taking medications or supplements of unclear origin without medical advice, particularly if kidney function is already impaired.
Conclusion
The kidney function markers most commonly assessed in blood tests include Creatinine, Urea or BUN, and eGFR. Creatinine reflects the amount of a waste product in the blood, while eGFR provides a more direct estimate of the kidneys’ filtering capacity. Urea or BUN can provide additional information but should not be interpreted in isolation.
An abnormal test result does not necessarily mean that kidney disease is present. Kidney function should be evaluated based on trends over time and interpreted together with urine test results, risk factors, and the individual’s overall health condition.
People with diabetes, high blood pressure, or other risk factors for kidney disease should undergo regular testing even if they have no symptoms. Early detection of declining kidney function gives doctors the opportunity to intervene and monitor the condition before it progresses to a more advanced stage.
