Creatinine – Reference Ranges, Indicators & Result Interpretation
Creatinine is an important marker of kidney function, with serum levels reflecting the health of these critical organs. Analysis of creatinine results provides information on possible kidney dysfunction and supports monitoring of treatment progress. Reference ranges for creatinine vary with age, sex, and body weight, so accurate interpretation is essential — it enables prompt diagnosis and appropriate medical intervention.
How to interpret your results
A single creatinine number rarely answers the question patients actually have: are my kidneys working? The body makes different amounts of creatinine depending on muscle mass, diet, age, and activity level, so a value that’s “high for one person” may be entirely normal for another. Blood creatinine can also stay within the normal range during the early stages of kidney disease and only rises once the condition is more advanced.
In practice, your creatinine is almost always paired with an estimated glomerular filtration rate (eGFR), which uses your creatinine value together with age, sex, height, and weight to estimate how fast your kidneys are filtering waste. eGFR is a more accurate measurement of kidney health than blood or urine creatinine alone. A single abnormal creatinine cannot diagnose a specific condition, so retesting is standard.
What “normal” looks like at your age
eGFR naturally declines with age even in people without kidney disease. Cleveland Clinic publishes the following average eGFR values by age range:
| Age (years) | Average eGFR |
|---|---|
| 20-29 | 116 |
| 30-39 | 107 |
| 40-49 | 99 |
| 50-59 | 93 |
| 60-69 | 85 |
| 70 and over | 75 |
For adults, a normal eGFR is about 100 or higher; an eGFR between 60 and 100 suggests mild kidney damage with kidneys still functioning well; an eGFR below 60 may indicate chronic kidney disease (CKD). The NHS uses a similar threshold: healthy kidneys should be able to filter more than 90 ml/min, and a lower rate may point toward CKD. If your clinician suspects a kidney problem, they will likely order additional tests — including eGFR and a urine ACR — even when creatinine looks unremarkable.
BUN/creatinine ratio — what it means when reported together
When creatinine is run as part of a basic metabolic panel (BMP) or a comprehensive metabolic panel (CMP), it sits next to another waste-product marker called blood urea nitrogen (BUN). Looking at the two together — sometimes expressed as a BUN/creatinine ratio — can help clinicians identify the cause of a kidney problem rather than just confirm that one exists.
Why pairing the two markers helps
Creatinine and BUN are both waste products that healthy kidneys filter out, but they don’t move in lockstep. Creatinine is produced when you use your muscles and small amounts of muscle tissue break down, so it tracks muscle activity and turnover. Because the two markers respond to different inputs, their pattern together gives clinicians more information than either number alone.
What to do with a flagged ratio
A BUN/creatinine ratio outside your lab’s reference range is a prompt for context, not a diagnosis. Useful questions to raise with your clinician:
- Could dehydration at the time of the draw have shifted the ratio?
- Were you on a high-meat diet in the day before the test?
- Are you taking medications or supplements that can affect kidney markers?
- Should this be repeated alongside an eGFR and urine ACR for a fuller picture?
Cached references describe the comparison qualitatively rather than publishing universal numeric cutoffs, so treat any specific ratio threshold your lab prints as a lab-specific reference range.
Creatinine vs eGFR vs creatinine clearance — three numbers, one kidney
Creatinine is the raw measurement; eGFR and creatinine clearance are calculations built on top of it. The table below shows what each measures, what sample it needs, and when each is most useful:
| Dimension | Serum creatinine | eGFR | Creatinine clearance |
|---|---|---|---|
| What it measures | Concentration of creatinine in your blood | Estimated milliliters of waste your kidneys filter per minute | Filtration estimated directly by comparing blood and urine creatinine |
| Sample type | Blood draw | Blood draw, with age/sex/height/weight applied to the equation | Blood draw plus a 24-hour urine collection |
| How it’s derived | Measured directly on the blood sample | Calculated from serum creatinine plus age, weight, height, and sex | Calculated from the blood-to-urine creatinine comparison rather than an equation |
| What clinicians use it for | One input into kidney assessment; varies by muscle mass, diet, age, and activity | The more accurate everyday measure of kidney health, used to track kidney disease over time | Useful when muscle mass is unusually high or low — substantial muscle bulk, muscle loss from age or illness, or limb loss |
When each number is most useful
Because creatinine production is tied to muscle mass, the same lab value carries different meaning in a 25-year-old bodybuilder, a frail 80-year-old, or someone who has lost a limb. Creatinine alone isn’t the best way to check how well your kidneys are working because production varies so much from person to person. Clearance is most useful when muscle mass is unusually high or low.
Where urine ACR fits in
Your kidneys filter waste, but they also retain useful proteins like albumin. A urine ACR (sometimes called a urine albumin-creatinine ratio or UACR) checks whether the filter is leaking albumin, which can be an early sign of kidney damage that creatinine alone may miss. ACR alongside eGFR gives a more complete picture than either number on its own.
CKD stages and what your creatinine result means over time
If repeat testing confirms a low eGFR, your results are mapped to a stage of chronic kidney disease (CKD) to guide treatment and follow-up frequency. Staging is based on eGFR rather than raw creatinine because eGFR adjusts for the body-composition factors that move creatinine around.
The NHS publishes these eGFR-based stages:
| Stage | eGFR (ml/min) | Notes |
|---|---|---|
| G1 | Above 90 | Normal eGFR, but other tests show kidney damage |
| G2 | 60-89 | Slightly reduced, with other signs of kidney damage |
| G3a | 45-59 | Moderate reduction |
| G3b | 30-44 | Moderate reduction |
| G4 | 15-29 | Severe reduction |
| G5 | Below 15 | Kidneys have lost almost all function |
For each stage, a higher number indicates more severe disease. Cleveland Clinic aligns at the entry threshold: an eGFR below 60 may indicate CKD.
What staging changes about your care
The stage shapes how often you’ll be retested and what other testing your clinician adds. The NHS notes that the stage helps decide the best treatment and how often monitoring tests should be done. Staging is also typically paired with an ACR stage (A1-A3) for a more complete risk picture.
Cleveland Clinic adds that if early kidney disease shows up, providers often order more frequent follow-up tests, may prescribe antihypertensives to manage high blood pressure, and may refer you to a kidney specialist (nephrologist). A single mildly low eGFR is not a diagnosis — confirmation requires repeat testing over time, plus urine testing for albumin, before a CKD stage is assigned.
What can throw off a creatinine result (non-kidney causes of abnormal values)
A number of things can move blood creatinine without your kidneys being involved. Knowing this list helps you and your clinician separate a true kidney signal from background noise.
Things that can raise creatinine
- Dehydration at the time of the blood draw
- Muscle disorders and injuries, including muscular dystrophy
- Intense exercise in the days before the test
- A diet high in meat in the 24 hours before the test
- Certain pregnancy-related health problems
- True kidney conditions — infection, poor blood flow to the kidneys, urinary blockage, or kidney failure — and systemic conditions that affect the kidneys such as heart failure or diabetes
Things that can lower creatinine
Low blood creatinine is uncommon, but when it happens it usually points to reduced muscle bulk or nutritional status rather than kidney disease:
- Malnutrition
- A long illness that depletes muscle
- A nerve disorder affecting muscle
- Age-related muscle loss
- Serious liver disease
Medications and supplements
Certain medicines and supplements can also affect creatinine, which is why your provider will ask you to share everything you’re taking before the test — including over-the-counter products. Don’t stop any medication on your own; if a drug is influencing your levels, the clinician will decide whether and how to adjust.
Frequently asked questions
What is creatinine in a blood test?
Creatinine is a normal waste product your body makes when you use your muscles and small amounts of muscle tissue break down. Your kidneys filter it out of your blood and into your urine, so blood levels rise when the kidneys aren’t filtering well.
What does it mean if creatinine is high?
A high blood creatinine can signal kidney disease or injury — including infection, poor blood flow to the kidneys, a urinary blockage, or kidney failure — or a condition like heart failure or diabetes that affects the kidneys. It can also be raised by dehydration, muscle injuries, intense exercise, a high-meat diet, or certain pregnancy issues.
What does it mean if creatinine is low?
Low blood creatinine is uncommon. When it occurs, it usually reflects reduced muscle bulk or nutrition rather than kidney disease — for example malnutrition, a long illness, a nerve disorder, age-related muscle loss, or serious liver disease.
What is the BUN/creatinine ratio?
It’s the relationship between blood urea nitrogen and creatinine, both measured on a CMP or BMP, that can help your clinician identify the cause of a kidney problem rather than just confirm one exists. See the dedicated BUN/creatinine ratio section above for what to do with a flagged result.
What’s the difference between serum creatinine, creatinine clearance, and eGFR?
Serum creatinine is the raw blood level; eGFR is the more accurate everyday measure of filtration calculated from your creatinine plus age, sex, height, and weight; creatinine clearance compares blood and urine creatinine and is most useful when muscle mass is unusual. See the comparison table above for a side-by-side breakdown.
When should I worry about a low creatinine?
A low creatinine isn’t typically a kidney emergency, but it can flag an underlying problem with muscle loss, long illness, malnutrition, or liver disease worth discussing with your clinician. Bring it up especially if it’s a new finding alongside fatigue, weight loss, or appetite changes.
Can diet, exercise, or supplements change my creatinine?
Yes. A high-meat diet in the 24 hours before testing can temporarily raise creatinine, which is why some clinicians ask patients to avoid meat for 24 hours before the draw. Intense exercise can also push levels up. Certain medications and supplements affect results too, so share your full list with your provider.
When to talk to your doctor
Creatinine results are best interpreted with your clinician, especially when paired with eGFR, urine testing, and your medical history. Consider reaching out promptly if:
- Your blood creatinine is flagged high or low on a repeat test, not just a single draw
- Your eGFR is below 60, or has dropped meaningfully since your last test
- You have known risk factors for kidney disease — diabetes, high blood pressure, heart disease, family history of kidney disease, age over 50, smoking, or obesity — and haven’t had recent kidney testing
- You take long-term medications that can affect the kidneys, such as lithium, omeprazole, or NSAIDs, and are due for monitoring
- You notice possible kidney symptoms: swelling in the hands, feet, or ankles, foamy or bloody urine, a clear change in how often you urinate, dry or itchy skin, persistent fatigue, muscle cramps or spasms, nausea or vomiting, shortness of breath, sleep problems, or trouble thinking clearly
- You have persistent symptoms such as weight loss, poor appetite, swollen ankles, tiredness, or peeing more than usual — particularly at night — that haven’t been explained
If your clinician confirms a problem on retesting, expect them to add an eGFR and a urine ACR to the workup and to discuss next steps based on the CKD stage your results fall into. Early-stage CKD often has no symptoms, which is why routine testing matters more than waiting for something to feel wrong.
References
- MedlinePlus (U.S. National Library of Medicine, NIH)
- Cleveland Clinic
- NHS (UK National Health Service)