CreatininevsCystatin C

Creatinine and cystatin C both estimate how well your kidneys filter blood. The difference is where each one comes from: creatinine from muscle, cystatin C from almost every cell. That single fact explains when they agree, when they split, and why the most accurate eGFR uses both.

What is the difference between cystatin C and creatinine?

Creatinine is a breakdown product of creatine, a compound stored in muscle. The more muscle you carry, the more creatinine you make, so the blood level reflects both kidney function and body build. Labs adjust for this with age and sex in the eGFR equation, but those are averages; your own muscle mass can sit far from them. The creatinine marker page covers its ranges.

Cystatin C is a small protein released at a fairly steady rate by nearly all the body's cells and filtered by the kidneys. Because muscle does not drive it, it avoids creatinine's biggest error. It has its own: in a study of 8,058 adults in the Netherlands, older age, male sex, greater weight and height, current smoking and higher C-reactive protein were each linked to higher cystatin C after allowing for kidney function measured by creatinine clearance. ⁵ The cystatin C marker page covers its ranges.

Both markers are turned into an eGFR with the CKD-EPI equations. Since 2021, US labs have moved to versions that leave out race, on the recommendation of the national kidney task force. ⁶ Without race, the creatinine-only equation is less accurate for some groups than the combined creatinine and cystatin C equation, which kept differences between groups smaller. ² That finding is one reason the task force called for cystatin C to be easier to get. ⁶

Cystatin C and creatinine side by side

Why the combined eGFR is the most accurate

Each marker's errors come from different sources, so averaging them cancels some of both. When the equations were developed, the combined creatinine and cystatin C equation performed better than either marker alone, with the share of estimates more than 30% away from measured GFR falling to 8.5%, against 12.8% for creatinine alone and 14.1% for cystatin C alone. ⁴ The 2021 race-free CKD-EPI equations now used by US labs found the same: the creatinine plus cystatin C equations were more accurate than creatinine alone and showed smaller differences between groups. ²

The 2024 KDIGO chronic kidney disease guideline, the main international guideline, keeps creatinine as the first test and advises measuring cystatin C, and using the combined equation, in situations where the creatinine-based eGFR is less accurate and the result will affect a clinical decision. ¹ The US kidney task force on race and eGFR also recommended wider and faster access to cystatin C testing. ⁶

Cystatin C is also a stronger risk signal

A meta-analysis of 11 general-population studies covering 90,750 people found that 13.7% had an eGFR under 60 by cystatin C, against 9.7% by creatinine. More important for a reader with a borderline result: people whose eGFR was moved up by cystatin C had lower risks of death and kidney failure, and those moved down had higher risks. Cystatin C, alone or with creatinine, strengthened the link between eGFR and those outcomes. ³

When cystatin C and creatinine disagree

A gap between the creatinine eGFR and the cystatin C eGFR is common, and the direction of the gap usually points to the reason.

Creatinine eGFR low, cystatin C eGFR normal

This is the pattern in people with a lot of muscle, people who ate a large portion of cooked meat before the blood draw, and people taking creatine. Each raises creatinine without touching filtering. ¹ Some medicines, such as the antibiotic trimethoprim, raise creatinine by blocking its release into urine, also without changing GFR. ^{1 7} Here the cystatin C result, or the combined eGFR, is usually the better read. LifeFrom's creatine guide covers the supplement side of this.

Creatinine eGFR normal, cystatin C eGFR low

This is the pattern to take more seriously. It shows up in people with little muscle, such as older or frail adults, people with an amputation, long illness, liver cirrhosis or a very low body weight, where a low creatinine hides reduced filtering. ¹ It can also come from things that raise cystatin C itself: inflammation, high-dose steroids, an overactive thyroid, smoking or obesity. ^{1 5} In the large meta-analysis, people reclassified to a lower eGFR by cystatin C had higher risks of death and kidney failure. ³

Both low

Agreement makes reduced kidney function more likely. Chronic kidney disease is confirmed when eGFR stays under 60, or kidney damage markers such as urine albumin to creatinine ratio stay raised, for more than three months. ¹ See chronic kidney disease for the stages.

Thyroid disease

Thyroid hormone moves the two markers in opposite directions, which can make the gap confusing. KDIGO lists both underactive and overactive thyroid among the things that shift cystatin C apart from kidney function. ¹ In a small study of people with thyroid disease, an underactive thyroid lowered cystatin C while creatinine rose, and an overactive thyroid did the reverse. ⁸ If you have hypothyroidism or hyperthyroidism that is not yet controlled, a single eGFR from either marker is less reliable.

Which one should you ask about?

• For routine screening: creatinine with eGFR is the standard, together with a urine albumin to creatinine ratio if you have diabetes or high blood pressure. ¹
• If your eGFR sits near the chronic kidney disease line of 60 and a diagnosis or medicine decision depends on it: ask whether a cystatin C or combined eGFR would help. ¹
• If you are very muscular, take creatine, or eat a high-meat diet: cystatin C can show whether a low creatinine eGFR is real.
• If you are older, frail, underweight or have lost a limb: cystatin C can show whether a normal creatinine eGFR is hiding reduced filtering.
• If a medicine is dosed by kidney function: a combined eGFR gives the more accurate number. ¹

Whichever marker you have, compare it with your own earlier results rather than reading one number alone, and keep the same lab where you can. A single eGFR can shift from one test to the next even when the kidneys have not changed, and KDIGO notes that a cooked meat meal in the hours before the draw can raise creatinine. ¹ The eGFR marker page and the eGFR calculator explain the estimate, and how to read your renal panel puts creatinine beside blood urea nitrogen and the BUN to creatinine ratio.

Creatinine reflects your kidneys and your muscles, cystatin C reflects your kidneys and a few other things, and when the two disagree, the reason is usually your body build, a medicine, or inflammation. A clinician deciding on a diagnosis or a medicine dose gets the most accurate estimate from both together.

Free in LifeFromAdd the lab results you already have, and LifeFrom reads each marker next to the others it moves with, in plain language, with the sources it used. Free, on the web or in the app.

Start on the web Download the app