Cystatin C: A Better Look at Kidney Function?

When I check how well your kidneys are working, I usually start with two basic tests – creatinine and the estimated glomerular filtration rate (eGFR). Both are solid, well-worn tools. Most of the time they tell me exactly what I need to know.

But there's a third test that doesn't get nearly enough credit – Cystatin C. I think it deserves a little more airtime.

What is Cystatin C?

Cystatin C is a small protein made by cells all over your body. Your kidneys filter it out of your blood. When kidney function starts to slip, cystatin C levels creep up.

Why bother checking it if creatinine already works?

Here’s the catch with creatinine. It’s a bit of a people-pleaser. Its levels can shift based on muscle mass, diet, age, and several other factors that have little to do with your kidneys. A bodybuilder and a marathon runner may have very different creatinine levels despite having equally healthy kidneys, and creatinine doesn’t always tell us why.

Hydration can also muddy the picture. Because many people fast before having blood work (I rarely require fasting, by the way), they may forget to drink enough fluids. Dehydration can concentrate the blood and reduce blood flow to the kidneys, temporarily raising creatinine and lowering the eGFR. The result? Your kidney function may look worse on paper than it actually is.

Cystatin C, on the other hand, is much less influenced by muscle mass and appears to be less affected by hydration status. That makes it a useful complement to creatinine and, in some situations, a more reliable marker of kidney function.

Who may benefit from this test?

Cystatin C earns its keep when the picture is murky. Say, your creatinine and eGFR are borderline, or the numbers just don’t seem to match the clinical picture. It can also be especially useful when muscle mass is well outside the average, whether you have considerably more or less muscle than most people. That’s exactly where creatinine can become harder to interpret.

To be clear, cystatin C isn’t a universal upgrade, and I’m certainly not declaring creatinine obsolete. It’s simply another useful tool in the toolbox. One that can sometimes give me a clearer picture of your kidney function.

Bottom line

Next time you're in (in-person at my downtown St. Pete location or virtually at www.remedicus.com), ask me whether cystatin C makes sense for you. It's one extra vial, and sometimes that's all it takes to get real clarity on how your kidneys are holding up.

Written by Dr. Elias Lu