Creatinine: what it is, what it tells you and why we test it

Creatinine is a waste product your muscles make every day and your kidneys clear. Its level in your blood helps show how well your kidneys filter. But it also depends on how much muscle you have.

Why we test it

Your kidneys filter your blood all day. If they filter less, creatinine builds up and the number rises.

Creatinine is used to calculate eGFR, the estimate of how well your kidneys filter. For your kidneys, it is the number that matters most.

Kidneys can slow down for years without symptoms. A blood test picks it up in time.

What it tells you, and what it does not

If it is high, your kidneys may be filtering less. But often the reason is something else: more muscle, a creatine supplement or too little water.

If it is low, it usually means little muscle. The kidneys are rarely involved.

And the same number does not mean the same thing at 30 and at 80. That is why eGFR matters too: it takes age and sex into account.

That is why it is read together with other tests:

• eGFR: How well your kidneys filter, for your age
• Urine albumin: Whether your kidneys let protein through
• Potassium: Whether your kidneys can clear it
• Glucose: Diabetes is a common cause of kidney damage

Normal creatinine levels

The lab decides what is normal. It is usually 0.51 to 0.95 mg/dL for women and 0.67 to 1.17 for men. Your own report may print different limits.

Men usually have more muscle, so their number is higher. For the same reason it is often higher in people who do a lot of sport.

Age does not change the limits on your report, but at 80 the same number means kidneys that filter less than at 30.

In the Rosa report you also see your optimal range and your eGFR, calculated for your age.

What your result means

Find the row for your result, in mg/dL. Which values are dangerous? For your kidneys, eGFR is what counts: below 60 should always be seen by a doctor.

• Below your report’s limit, Women: usually below 0.51, Men: usually below 0.67: Low. It usually means little muscle. On its own it does not show your kidneys are fine. If you have lost weight or strength without trying, talk to your doctor.
• Within your report’s limits, Women: usually 0.51 to 0.95, Men: usually 0.67 to 1.17: Normal. Usually fine. Check your eGFR too, which takes your age into account. In the Rosa report you also see whether you are in your optimal range.
• Above your report’s limit, Women: usually above 0.95, Men: usually above 1.17: High. It can come from muscle, creatine or too little water, or from kidneys that filter less. If your eGFR is below 60 or your report flags albumin in your urine, talk to a doctor. Otherwise, repeat the test rested and well hydrated.

If your creatinine is high or low

• High creatinine: Muscle, creatine or too little water are often behind it. Sometimes the kidneys filter less.
• Low creatinine: It usually means little muscle. On its own it does not show your kidneys are fine.

What can skew the result

A few things change the number for a while. It helps to know them before you read it.

• Creatine supplements: They push it up a little, even with healthy kidneys. Tell your doctor.
• Hard exercise: A tough workout in the days before can raise it for a short while. Arrive at the blood test rested.
• Too little water: If you are dehydrated, the number rises. You can drink water even while fasting.
• Lots of meat or protein: A meat meal can raise it for many hours: best to have the blood test fasting. A very high protein diet can also push it up a little.

When to worry

Talk to a doctor if:

• Your eGFR is below 60.
• Your report flags albumin in your urine.
• Your creatinine rises from one test to the next.
• Your ankles are swollen.
• You pass much less urine than usual: see a doctor the same day.

With Rosa, an eGFR below 60 or a urine albumin-to-creatinine ratio of 30 mg/g or more is always flagged to a doctor.

Frequently asked questions

What is the difference between creatinine and eGFR? Creatinine is the number measured in your blood. eGFR is an estimate of how well your kidneys filter, calculated from creatinine, age and sex. For your kidneys, eGFR matters more.

What is creatinine clearance? Another way to estimate how well your kidneys filter, usually by collecting your urine for 24 hours. Today eGFR is used more often, because it only needs a blood test.

Do normal values change with age? The limits on your report usually do not. But with age the kidneys filter a little less and muscle shrinks, so the same number reads differently. eGFR takes age into account.

Do I need to fast for a creatinine test? It is better. A meat meal can raise it for many hours, and other tests in the same draw need fasting anyway. You can drink water.

How often should it be checked? For people who feel well there is no fixed rule: Rosa measures it twice a year. If the number is out of range, it is usually checked again after about three months, or sooner if your doctor says so.

Sources

• KDIGO clinical practice guideline for the evaluation and management of chronic kidney disease. Kidney International, 2024. PMID 38490803
• Inker and colleagues, new equations to estimate kidney filtration from creatinine and cystatin C. New England Journal of Medicine, 2021. PMID 34554658
• Groothof and colleagues, creatinine, cystatin C, muscle mass and mortality in two population cohorts. Journal of Cachexia, Sarcopenia and Muscle, 2024. PMID 38898741
• Nair and colleagues, effect of a cooked meat meal on serum creatinine and eGFR. Diabetes Care, 2014. PMID 24062331