Understanding Your Kidney Function Tests

Kidney function tests are a group of blood and urine tests used to assess how well your kidneys are working. They are among the most important tests in medicine because chronic kidney disease often progresses silently, with no symptoms until function is significantly impaired. Routine kidney testing allows the disease to be detected and treated long before damage becomes severe.

This article focuses on the blood-based kidney function tests, including creatinine, blood urea nitrogen (BUN), estimated glomerular filtration rate (eGFR), cystatin C, and creatinine clearance. Together, these tests provide a detailed picture of kidney health.

The reference range that applies to your result is the one printed on your laboratory report, not the typical ranges shown here. Reference ranges vary between laboratories based on the equipment used, the population tested, and individual factors such as age, sex, and pregnancy status. Always compare your result to the reference range printed on your own report, and discuss any abnormal result with your doctor.

What do the kidneys do?

The kidneys are two bean-shaped organs in the back of the abdomen that perform many essential jobs:

Filter waste products from the blood and remove them in urine
Balance fluids and electrolytes such as sodium, potassium, and calcium
Control blood pressure through the regulation of fluid balance and the production of certain hormones
Maintain acid-base balance by regulating bicarbonate and excreting acid in urine
Produce hormones including erythropoietin, which signals the bone marrow to make red blood cells, and active vitamin D
Remove medications and toxins from the body

Each kidney contains roughly one million tiny filtering units called nephrons. The collective filtering capacity of all nephrons is called the glomerular filtration rate (GFR), the most important single measure of kidney function.

Why are kidney function tests done?

Kidney function tests are ordered for many reasons:

Routine screening. Kidney tests are part of most routine blood test panels, including the basic metabolic panel and comprehensive metabolic panel. They are particularly important for people with risk factors such as diabetes, high blood pressure, heart disease, or a family history of kidney disease.
To investigate symptoms. Swelling in the legs or face, foamy urine, decreased urine output, fatigue, nausea, and unexplained high blood pressure can all prompt kidney testing.
To monitor known kidney disease. Patients with chronic kidney disease have regular testing to track changes over time.
To monitor medications. Many medications — including ACE inhibitors, diuretics, NSAIDs (ibuprofen and similar), antibiotics, contrast dyes, lithium, and chemotherapy — can affect the kidneys, and testing is used to monitor their effects.
Before procedures. Kidney function is checked before surgery, before contrast-enhanced imaging studies (CT scans with dye), and before starting many medications, since impaired kidney function may require dose adjustments or alternative approaches.
To assess transplant kidney function. Patients who have received a kidney transplant have frequent kidney function testing to monitor the transplanted kidney.

How are the tests performed?

Most kidney function tests are performed using a small blood sample drawn from a vein in the arm. No fasting is required for most kidney tests, though some doctors prefer fasting samples when kidney tests are part of a larger panel. Creatinine clearance is an exception — it requires both a blood sample and a 24-hour urine collection (described below).

The main kidney function tests

Creatinine

Creatinine is a waste product produced by normal muscle activity. The kidneys filter creatinine out of the blood and excrete it in urine. Because creatinine production is fairly steady from day to day, blood creatinine levels reflect how well the kidneys are filtering — when filtering slows, creatinine builds up.

A typical reference range for adults is 0.84–1.21 milligrams per decilitre (mg/dL), but this range varies by age, sex, and body size. People with greater muscle mass typically have higher baseline creatinine levels. Older adults and people with reduced muscle mass typically have lower creatinine.

Causes of high creatinine:

Acute kidney injury — sudden loss of kidney function from causes such as dehydration, low blood pressure, certain medications, contrast dye, urinary tract obstruction, or severe infection
Chronic kidney disease, most commonly caused by diabetes and high blood pressure
Heart failure (which reduces blood flow to the kidneys)
Severe muscle injury (rhabdomyolysis), which releases large amounts of creatinine into the blood
Certain medications including some antibiotics, NSAIDs, ACE inhibitors, and chemotherapy
Diet very high in cooked meat (a small temporary rise)

Causes of low creatinine:

Reduced muscle mass, common in older adults
Severe liver disease
Pregnancy (a normal physiologic decrease)
Prolonged bed rest

A single creatinine result is most useful when compared to your previous results. A small upward trend over time can be more informative than a single value within the normal range.

Estimated glomerular filtration rate (eGFR)

The estimated glomerular filtration rate is a calculation that estimates how well the kidneys are filtering blood, expressed in millilitres per minute per 1.73 square metres of body surface area (mL/min/1.73m²). It is reported alongside creatinine in most modern laboratory reports and provides a more clinically useful measure of kidney function than creatinine alone.

The eGFR is calculated from the blood creatinine result, age, and sex. Older calculation methods also adjusted for self-reported race, but most current laboratory equations no longer include race, since this practice was found to introduce bias and reduce accuracy.

Typical eGFR values and chronic kidney disease (CKD) staging:

90 or higher — normal or high kidney function (CKD stage 1 if other signs of kidney damage are present, otherwise normal)
60–89 — mildly decreased (CKD stage 2 if other signs of kidney damage are present)
45–59 — mildly to moderately decreased (CKD stage 3a)
30–44 — moderately to severely decreased (CKD stage 3b)
15–29 — severely decreased (CKD stage 4)
Below 15 — kidney failure (CKD stage 5); typically the threshold for considering dialysis or transplantation

The diagnosis of chronic kidney disease requires either an eGFR below 60 sustained for at least three months, or other evidence of kidney damage (such as protein in the urine) for at least three months.

The eGFR is most accurate when kidney function is stable. In people whose kidney function is changing rapidly — such as in acute kidney injury — the eGFR can lag behind the true filtering rate by hours or days.

Blood urea nitrogen (BUN)

Blood urea nitrogen measures the amount of nitrogen in the blood that comes from urea, a waste product of protein breakdown. The kidneys filter urea out of the blood and excrete it in urine.

A typical reference range for adults is 6–20 mg/dL.

BUN reflects kidney function but is also influenced by hydration status, dietary protein intake, gastrointestinal bleeding, and medications. For this reason, BUN is less specific to kidney function than creatinine and is most useful when interpreted alongside creatinine.

The BUN-to-creatinine ratio can suggest specific causes of kidney problems:

A high BUN:creatinine ratio (above about 20:1) often suggests dehydration, decreased blood flow to the kidneys, or gastrointestinal bleeding rather than primary kidney damage. This is sometimes called a “prerenal” pattern because the problem is “before” the kidney itself.
A normal BUN:creatinine ratio (about 10–20:1) with both values elevated typically suggests damage to the kidney itself.
A low BUN:creatinine ratio (below 10:1) may be seen in liver disease, low protein intake, or conditions that lower BUN production.

Cystatin C

Cystatin C is a small protein produced by all cells in the body and filtered by the kidneys. Like creatinine, it builds up in the blood when kidney filtering is impaired. Unlike creatinine, cystatin C is not significantly affected by muscle mass, age, sex, or diet, which makes it more accurate in certain populations:

Older adults with reduced muscle mass
People with very high or very low body weight
People with conditions that affect creatinine production
Patients in whom an accurate eGFR is particularly important, for example, before starting medications that require precise dosing based on kidney function

A typical reference range for adults is approximately 0.6–1.3 mg/dL. Like creatinine, cystatin C can be used to calculate an estimated GFR (sometimes called eGFRcys), and combined eGFR equations using both creatinine and cystatin C are now considered the most accurate non-invasive measure of kidney function.

Cystatin C is not yet routinely included in all kidney function panels but is increasingly available, particularly when creatinine-based estimates are likely to be inaccurate.

Creatinine clearance

Creatinine clearance is a more direct measurement of kidney filtering function. It is calculated by comparing the amount of creatinine in your blood with the amount of creatinine excreted in your urine over a defined period — typically a 24-hour urine collection.

The test requires you to collect every drop of urine you produce over a 24-hour period in a special container provided by your laboratory, and to have a blood sample drawn during that period. The laboratory measures creatinine in both samples and calculates the clearance.

A typical reference range for adults is approximately 88–128 mL/min for women and 97–137 mL/min for men, decreasing somewhat with age.

Creatinine clearance was once the standard way to measure kidney function but has largely been replaced by the eGFR for routine use, since the eGFR is calculated from a single blood test and does not require a 24-hour urine collection. Creatinine clearance is still occasionally used in specific situations — for example, when very precise measurement of kidney function is needed for medication dosing or research.

Other kidney-related tests

Several other tests are commonly ordered alongside the main kidney function tests to provide a more complete picture:

Urine albumin (or urine albumin-to-creatinine ratio). Tests for the presence of small amounts of albumin in the urine, called albuminuria. This is one of the earliest signs of kidney damage, particularly in people with diabetes or high blood pressure, and often appears before the eGFR drops. The urine test is sometimes the first warning that kidney damage is occurring.
Urinalysis. A general urine test that can detect protein, blood, glucose, infection, and abnormal cells in the urine.
Electrolytes (sodium, potassium, bicarbonate, chloride). Measured as part of the BMP and CMP and reflect the kidney’s role in fluid and acid-base balance.
Calcium, phosphorus, and parathyroid hormone (PTH). Often abnormal in chronic kidney disease, as the kidneys play a role in mineral and bone metabolism.
Hemoglobin. Frequently low in chronic kidney disease because the kidneys produce erythropoietin, the hormone that signals the bone marrow to make red blood cells.

What happens after kidney function testing?

If your results are within reference ranges, no further investigation is usually needed. If results are abnormal, the next steps depend on the severity, the trend over time, and the presence of any other findings.

Repeat the test. Mildly abnormal results, particularly when creatinine is borderline, may resolve with rest and rehydration. Repeating the test in days to weeks helps determine whether the abnormality is sustained.
Check urine for protein and blood. A urine albumin-to-creatinine ratio is one of the most useful follow-up tests when kidney function appears decreased.
Review medications. Your doctor will look for any medications, supplements, or contrast dyes that could be contributing to impaired kidney function and may stop or change them.
Check blood pressure and blood sugar. High blood pressure and diabetes are the two most common causes of chronic kidney disease, and controlling them is essential to slowing further damage.
Order imaging. A kidney ultrasound is the standard first imaging test and can detect obstructions, differences in kidney size, cysts, masses, and other structural abnormalities.
Consider cystatin C testing. If creatinine-based eGFR seems inaccurate or precise kidney function is needed, cystatin C may be added.
Refer to a nephrologist. Persistent CKD — particularly stage 4 (eGFR 15–29) and stage 5 (eGFR below 15) — typically warrants referral to a kidney specialist (nephrologist).
Consider a kidney biopsy. When the cause of kidney dysfunction is unclear, a kidney biopsy may be performed to obtain a tissue diagnosis.

Once chronic kidney disease is diagnosed, the goals of care are to slow progression, manage complications (such as anemia, bone disease, and high blood pressure), and prepare for advanced kidney support — dialysis or transplantation — if and when it becomes necessary.

Questions to ask your doctor

What is my eGFR, and what stage of kidney function does it represent?
How does my current result compare to my previous results?
Could any of my medications, supplements, or recent procedures (such as contrast dye) be affecting my kidneys?
Should I have a urine test for albumin?
Do I need a kidney ultrasound?
Should the test be repeated, and if so, when?
Are there changes I should make to my medications, fluid intake, or diet to protect my kidneys?
Should I be referred to a nephrologist?
If I have chronic kidney disease, what stage am I at, and what does that mean for my future treatment?
Are there medications I should avoid because of my kidney function?