What is creatinine?

Creatinine is one of the most common results on a kidney blood test. Here’s what it is, why it shows up on your labs, and what a high or low level can mean.

What is creatinine?

Creatinine is a waste product your body makes every day. Your muscles use a compound called creatine for energy, and as they do, they produce creatinine at a fairly steady rate. Healthy kidneys filter creatinine out of your blood and remove it in your urine, so the amount left in your blood is a useful clue to how well your kidneys are working.

Why is creatinine on my blood test?

Because your kidneys clear creatinine, the level in your blood rises when filtering slows down. That makes serum creatinine a simple, inexpensive marker of kidney function. On its own, though, the raw number is hard to read, which is why the lab also uses it to calculate your eGFR, a more standardized estimate of kidney function that accounts for your age and sex.

What makes creatinine go up or down?

A higher-than-expected creatinine can reflect reduced kidney function, but several other things move the number too:

• Muscle mass. Creatinine comes from muscle, so very muscular people tend to run higher, while people with low muscle mass (including many older adults) run lower.
• Dehydration and a large meat or high-protein meal can raise it temporarily.
• Certain medications can raise the measured level, some by genuinely affecting the kidneys, others simply by blocking how creatinine is secreted without changing true kidney function.
• Acute illness or sudden kidney stress can push it up quickly.

For all those reasons, one creatinine value is rarely the whole story. Doctors look at the pattern over time and pair it with other tests.

How does creatinine relate to my kidney health?

Think of creatinine as the raw measurement and eGFR as the interpretation. A rising creatinine usually means a falling eGFR. If your creatinine is high or climbing, your doctor will typically repeat the test, check a urine sample for protein, and look at your overall health to understand why.

This article is for general education and is not a substitute for personalized medical advice. Talk with your doctor or nephrologist about your specific situation.