renal function test lab draw with creatinine

Creatinine vs. Cystatin C: Which Test Measures PKD Kidney Health Better?

Aug 18, 2025

Labs can be confusing, especially when it comes to monitoring kidney function in polycystic kidney disease (PKD). As a PKD and kidney nutrition expert, I get a lot of questions about interpreting these results, such as:

“What’s the difference between Creatinine and Cystatin C?”
“Which test is better for tracking kidney function in PKD?”
“Should I get both Creatinine and Cystatin C tested?”

Keep reading, I'll answer those questions for you, I’ll break down the similarities and differences between these two important kidney markers, explain how they relate to your eGFR results, and help you understand when a Cystatin C test might give a clearer picture of your kidney health.

Understanding GFR: A Look at Kidney Function

GFR stands for Glomerular Filtration Rate. It is an estimate of how well your kidneys are functioning and filtering fluid and waste. Think of GFR as a percentage on a scale from 0 to 100%. Anything below 60% indicates that your kidneys aren't filtering as well as they should be, suggesting decreased kidney function.

I want to clarify what eGFR means. eGFR is the abbreviation for Estimated Glomerular Filtration Rate, the "e" in eGFR stands for estimated. Know this, on your labs report and in most articles eGFR is often written as GFR - they are the same thing.

The standardized way to calculate eGFR involves measuring the level of creatinine waste in your blood and plugging that amount into an equation. If you want to see this in action, check out the National Kidney Foundation’s eGFR Calculator and plug in your latest creatinine lab results. In general, the higher the creatinine waste the lower kidney function is.

There is a more precise way to measure GFR—called mGFR (measured GFR)—but it’s time-consuming, expensive, and mainly used in research. So, when you hear “GFR,” it’s almost always an estimate (eGFR), not the gold-standard mGFR.

Estimated GFR doesn’t mean inaccurate. But it does explain why your numbers can bounce up or down a little without indicating big changes. External factors other than the kidneys can impact those numbers—more on that shortly.

Similarities Between Creatinine and Cystatin C

How Creatinine and Cystatin C are similar:

Both measure waste in blood: Creatinine and Cystatin C are both waste products filtered by the kidneys and can be measured with blood tests.
Both have standard reference ranges: Each has a "normal" range that appears on your lab results. This range represents how much is typically found when the kidneys are functioning at full capacity and aren't injured or impaired.
Both can indicate kidney damage: If either Creatinine or Cystatin C goes above the standard range, it may signal kidney damage or decreased kidney function.
Both are used to calculate eGFR: Either waste marker can be used to calculate your estimated GFR, they are kidney function markers.

Differences Between Creatinine and Cystatin C

While both Creatinine and cystatin C are are waste that can be used estimate kidney function, they differ in what can impact and influence their levels. Creatinine comes primarily from muscle breakdown and can be affected by muscle mass and diet, while cystatin C is made cells in the body and is much less influenced by those factors. Let's look a little closer at each!

What is Creatinine?

Creatinine is a waste product primarily generated from the normal wear and tear of your muscles. It's released into the bloodstream and then filtered out by the kidneys. When your kidneys are working at full function they are able to filter out almost all creatinine waste and get rid of it in your urine. Having an elevated blood level can indicate a loss of kidney function, a loss of some of that filtration system.

Diet can also impact creatinine. If you eat a lot of meat (especially red meat), your body converts creatine from that meat into creatinine. A dietary pattern heavy with animal meat can create an environment where the kidneys will have to filter in overdrive to keep up with removal of that waste. This is called hyperfiltration. It can also lead to higher creatinine levels in the blood.

Adult Creatinine Reference Ranges:

Male: 0.7 to 1.3 mg/dL
Female: 0.6 to 1.1 mg/dL

Creatine Supplements and Kidney Health

If you're taking (or thinking about taking) a creatine supplement, you might want to reconsider! Remember, creatine converts to creatinine in your body, adding more work for your kidneys. Animal studies have suggested that creatine supplementation may accelerate PKD kidney cyst growth. That’s the exact opposite of what you want to do with your PKD Diet and proactive PKD Nutrition.

What Can Influence Creatinine Levels?

There are factors that aren't related to the kidneys that can impact creatinine, and thus eGFR, results. They are worth knowing about, as no one wants to see elevated numbers even if they aren't related to kidney function!

Higher creatinine levels can result from:
- Higher muscle mass
- High protein diet
- Supplements, like carnitine
- Intense exercise before lab tests
- Illness or infection
- Certain medications, some that are commonly used with PKD like Tolvaptan (Jynarque), Losartan or Lisinopril ^{*please note, these meds are not causing kidney damage}

Lower creatinine levels can result from:
- Lower muscle mass, i.e. frail older adults
- Pregnancy
- Edema (fluid retention)
- Rapid weight loss
- Low protein diet

Why Creatinine is a Go-To Kidney Test

Creatinine lab tests are widely available, easy to measure, and have standardized results. They are relatively inexpensive and serve as a reliable indicator of kidney function, making them a universally used tool for diagnosing and monitoring kidney diseases.

What is Cystatin C?

Cystatin C is also a waste product—but unlike Creatinine, it’s produced by nearly all the cells in your body, not just muscle cells. It’s filtered by the kidneys and can be used to estimate how well your kidneys are functioning and filtering.

Cystatin C Reference Range:

Under 50 - 0.53-0.92 mg/L
Over 50 - 0.58-1.02 mg/L

What Can Influence Cystatin C Levels?

There are factors, much fewer that creatinine, that can impact lab results that aren't related to kidney function.

Higher Cystatin C levels can result from:
- Smoking
- Steroids
- Thyroid disorders
- Chronic inflammation, like with autoimmune diseases
- Thyroid dysfunction, Hyperthyroidism
Lower Cystatin C levels can result from:
- Thyroid dysfunction, Hypothyroidism
- Genetics

Cystatin C, Key Differences from Creatinine

Cystatin C levels are much less influenced by factors outside of actual kidney function. Cystatin C levels aren't affected by by sex, diet, muscle mass, and exercise. If you happened to notice, Cystatin C has reference ranges that aren't separated into male and female categories. Because Cystatin C isn't impacted as significantly by variables other than kidney function its seen as a more accurate estimated of kidney function for certain populations and individuals.

Of note, Cystatin C is believed to be more reliable, especially early on, with estimating kidney function and evaluating risk for kidney disease related to diabetes and/or hypertension. Some studies have show Cystatin C to provide a better marker of kidney function with PKD as it correlates better with total kidney volume (TKV) than creatinine.

When to Ask For a Cystatin C Test

Cystatin C provided a better estimate of kidney function:

With obesity, BMI > 40
For very muscular individuals, or weightlifters
When there are high levels of protein in urine

According to research, Cystatin C is not affected by factors like infections and medications that impact creatinine levels. Additionally, Cystatin C-based GFR calculations are often more accurate for the very young because they are not biased due to muscularity, and more accurate for the elderly because they are not biased due to frailty.

Why Isn't Cystatin C Tested More Often?

Even though Cystatin C could be considered more accurate, given it is less impacted by factors outside of the kidneys, Cystatin C testing isn't as common as creatinine testing for two main reasons: cost and availability.

Cystatin C testing is more expensive compared to creatinine testing The majority of lab centers are able to test for creatinine; however testing for Cystatin C is more specialized and not available everywhere.

That being said, if you have any of the factors that might affect creatinine levels (like high muscle mass or protein in urine), it might be worth discussing Cystatin C testing with your nephrologist.

When in doubt, talk to your doctor and/or renal dietitian if Cystatin C testing - or a combined GFR estimate - could give you a better picture of your kidney health and function.

What to Do When Creatinine and Cystatin C Results Differ?

Sometimes eGFR calculated using Cystatin C (GFRcys) and Creatinine (GFRcr) don’t match. Here’s my rule of thumb:

If the difference is under 15 points, use the average
If the difference is over 15 points, trust the Cystatin C result
Why? Because Creatinine is more likely being influenced by something other than your kidney health.

How to Track and Understand Your Kidney Labs

Know your numbers: Always review your lab results and understand what they mean. The National Kidney Foundation has an excellent online eGFR calculator where you can plug in your details and creatinine level to see your estimated GFR.
Track trends over time: Rather than focusing on a single result, look for patterns and trends in your labs. This gives a much clearer picture of your kidney function, what's working and what isn't.
Consider what might affect your results: If you see unexpected changes in your lab values, think about factors that might be influencing them outside of kidney function (like recent exercise, diet changes, or new medications).
Discuss Cystatin C with your doctor: If you're muscular, have a high BMI, or are spilling lots of protein spilling in your urine, ask your nephrologist for a Cystatin C tests. It might give a more accurate picture of your kidney function.
Stay hydrated: Proper hydration is crucial for accurate lab results, especially for creatinine levels. If you are dehydrated you will get a falsely elevated result.

Learn More: Master Your Labs for PKD

If you found this information helpful and want to learn more about understanding your PKD labs, I have a special resource for you! Master Your Labs for PKD is a self-paced course that walks you through everything you need to know about interpreting your lab results, including what can affect them outside of your kidney function.

This 8 module course course includes topics such as:

Kidney function labs
Bone health
Lipid labs
Anemia labs
And more!

Master Your Labs is entirely virtual and self-paced, allowing you to focus on the specific lab values that matter most to you. You'll gain lifetime access, so you can revisit any section whenever you receive new lab results.

In Summary

Both Cystatin C and Creatinine are valuable tools for monitoring kidney function with PKD. While Creatinine is more commonly used because it is readily accessible and cheaper to test, Cystatin C offers advantages since it is less affected by non-kidney factors.

Understanding the differences between these markers and knowing when to request specific tests can help you better understand, and take more control over your PKD Health management. Remember that knowledge is power when it comes to your health!

Have you had both Cystatin C and Creatinine tested? I'd love to hear about your experience in the comments below!

In Health,

Diana, The PKD Dietitian