Category: General
Abnormal EGFR level? Why Cystatin C is the next step
An abnormal eGFR value doesn’t automatically mean your kidneys aren’t working properly, especially not in athletic people with a lot of muscle mass. Creatinine is influenced by muscle mass, diet, and exercise, which can skew the calculated kidney function. The cystatin C test provides a more reliable and accurate picture of your actual kidney function.
Imagine this: you’ve had a blood test done to check on your health. You feel fit, exercise regularly, and watch what you eat. Then you get a call from bloodtesting.nl telling you that your eGFR level is abnormal and that we recommend follow-up testing with a cystatin C test. Your heart skips a beat. Problems with your kidneys? You certainly didn’t expect that.
This scenario is more common than you might think, especially among athletes and people with a lot of muscle mass. The good news is that in many cases, there’s no problem with your kidneys at all. The standard kidney function test can be influenced by factors that have nothing to do with your kidney health. That’s why, if your results are abnormal, we recommend an additional test: Cystatin C.
In this blog, we explain what eGFR and creatinine are, why these values sometimes provide an incomplete picture, and why cystatin C is the gold standard for a reliable measurement of your kidney function.
What is eGFR, and why is it important?
eGFR stands for estimated glomerular filtration rate. It sounds technical, but basically, eGFR indicates how well your kidneys are functioning. Your kidneys are your body’s filters: they purify your blood and remove waste products and excess fluid through your urine.
How do your kidneys work?
Your kidneys contain millions of tiny filtering units called glomeruli, which together filter about 180 liters of blood per day. The eGFR value indicates how many milliliters of blood are filtered per minute. In a healthy adult, this ranges between 90 and 120 ml/min/1.73 m².
Why do we measure eGFR?
Declining kidney function can go unnoticed for years, simply because your kidneys have a tremendous amount of reserve capacity. You won’t experience symptoms until more than 70% of your kidney function has been lost. That’s why it’s important to have your kidney function checked regularly. This is especially true if you have diabetes or high blood pressure, take pain relievers long-term, have a family history of kidney problems, are over 60 years old, or already have your blood tested regularly for your overall health.
Creatinine: The Standard Measurement and Its Limitations
To calculate your eGFR, we first measure the creatinine level in your blood. Creatinine is a waste product produced during the normal breakdown of muscle tissue. Your muscles constantly produce small amounts of creatinine, which travel through your blood to your kidneys, where they are filtered out.
Creatinine is the standard initial test for a few practical reasons: it’s cost-effective and quick to perform, well-standardized in laboratories worldwide, and reliable for monitoring over time in people with known kidney disease. For most people, creatinine therefore provides an accurate picture of kidney function. But there is one important point to note.
The muscle mass effect
Because creatinine is a byproduct of muscle tissue, the amount of creatinine in your blood is naturally influenced by your muscle mass. This isn’t a test error; it’s a biological characteristic of this marker.
Do you have a lot of muscle mass from strength training, sports, or a naturally muscular build? If so, you produce more creatinine, which can lead to a higher creatinine level in your blood—even if your kidneys are functioning perfectly. As a result, the eGFR calculation may show a lower value than your actual kidney function. This is particularly common among regular strength trainers and bodybuilders, active athletes with a lot of muscle mass, people with a naturally muscular build, and men, who on average have more muscle mass than women.
The reverse is also true: if you have low muscle mass, you produce less creatinine. This can occur in older people, people with muscle diseases, or after prolonged bed rest. In such cases, the creatinine level may appear normal, even though kidney function is actually impaired.
Other factors that affect creatinine
In addition to muscle mass, other factors also play a role. A meat-rich diet temporarily raises creatinine levels, while vegetarians and vegans often have lower levels. As we age, muscle mass naturally decreases, which affects the level, and men have, on average, 15 to 20% higher creatinine levels than women. Dehydration and recent intense exercise can also temporarily raise creatinine levels, and some medications interfere with its excretion.
Why We Call When an eGFR Is Abnormal
At bloodtesting.nl, we take our customers’ health seriously. When we see an abnormal eGFR value based on creatinine, we always call you personally to discuss it with you.
Context is crucial
Blood test results on paper don’t tell the whole story. We want to know if you’re training intensively or building your fitness, how much muscle mass you have, what kind of diet you follow, whether you’re taking medication, and if you have any symptoms that might indicate kidney problems. That information helps us determine whether your abnormal result is likely due to biological variation or whether further testing is truly necessary.
Preventing Unnecessary Worries
Seeing an abnormal kidney function result can be quite alarming. Many people immediately start Googling and panic about kidney failure, dialysis, and other serious scenarios. Through personal contact, we can provide immediate explanations and, where possible, reassure them. In many cases, especially among athletic individuals with a lot of muscle mass, there is actually no problem at all. The abnormal eGFR is simply the result of the natural effect of muscle mass on creatinine levels, not of reduced kidney function.
Identifying real problems early
Of course, it’s also possible that there is indeed something wrong with your kidneys. Early detection is essential, because kidney diseases can often still be treated effectively in the early stages. By talking with you, we can assess whether there are any warning signs that warrant further testing.
Appropriate, personalized advice
Based on our conversation and your personal situation, we’ll offer one of the following recommendations:
- Get a follow-up test with Cystatin C through us for a definitive answer about your kidney function
- Repeat the test after a few weeks, because levels can sometimes be temporarily abnormal due to dehydration or recent exercise
- Referral to your primary care physician in case of serious abnormalities or warning signs
- Reassurance: if the deviation can be clearly explained by your muscle mass and lifestyle
Cystatin C: the gold standard for accurate kidney function measurement
This is where cystatin C comes into play. It is an alternative biomarker for kidney function that does not have the limitations of creatinine and is considered the most reliable method for measuring your actual kidney function.
What is cystatin C?
Cystatin C is a small protein produced by virtually all cells in your body at a constant rate. Just like creatinine, it is filtered by your kidneys and removed from your blood. The key difference is that cystatin C production isn’t affected by your muscle mass, diet, age, or gender.
Why is cystatin C more reliable?
Whether you’re a muscular athlete or have little muscle mass, Cystatin C production remains constant. What you eat also has no effect on the level, and because production by all body cells is stable, cystatin C can detect early-stage kidney function decline sooner than creatinine. This makes the marker reliable across the entire range, from normal to reduced kidney function.
Numerous scientific studies show that cystatin C is more accurate than creatinine, especially in situations where creatinine can be misleading. International guidelines therefore recommend measuring cystatin C when there is uncertainty about kidney function. The most recent findings even suggest that a combination of both markers—creatinine and Cystatin C—provides the most accurate assessment.
When is cystatin C testing essential?
- Do you train intensively and does your eGFR appear lower than expected despite having a lot of muscle mass? Cystatin C provides the answer
- For borderline eGFR values, around 60 ml/min, where confirmation is desired
- In cases of extremely high or low muscle mass, such as in bodybuilders, older adults, or people with muscle diseases
- In the case of a vegetarian or vegan diet, where creatinine levels may be artificially low
- For important medical decisions, such as starting medication that may affect the kidneys or prior to surgery
- As part of follow-up for previous abnormalities, to monitor changes in your kidney function
- Simply to be on the safe side, if you want to know for sure how your kidneys are doing
The added value of our service
What sets bloodtesting.nl apart from a standard laboratory? We go beyond simply performing blood tests.
Personalized counseling
That phone call when your eGFR is abnormal isn’t just standard protocol—it’s genuine, personalized care. We take the time to understand your situation and explain what the results mean, without using medical jargon.
Expertise and Interpretation
We don’t just look at reference values; we put your results into the context of your specific situation. An eGFR of 65 ml/min means something different for a 70-year-old with diabetes than it does for a 30-year-old bodybuilder. We’ll discuss that nuance with you.
Preventing Unnecessary Follow-Up Procedures
By performing a cystatin C test in a timely manner, we can prevent you from having to see your primary care physician with a “abnormal kidney function” result when your kidneys are actually functioning perfectly well. This saves you from unnecessary worry, extra appointments with your primary care physician, and unnecessary referrals to a nephrologist.
Timely referral when necessary
On the other hand, if we see that there is indeed a reason for further medical evaluation, we’ll immediately point you in the right direction. We are not a substitute for medical care, but a valuable first step in understanding your health.
Speed and convenience
With us, you can easily schedule a cystatin C test online, with no waiting lists and no referral letter required. Within a few days, you can come in for a blood draw and receive your results quickly. With our personalized advice, you’ll have clarity about your kidney function within a week.
How does it work in practice?
You've already had a blood test with an abnormal eGFR
- You’ll get a call from us
- We’ll discuss your situation, lifestyle, and any symptoms you may have
- We recommend a cystatin C test to be sure
- You can easily order the test online through our website
- You’ll go to a blood draw location near you
- You’ll receive the results within a few days
- We’ll discuss the results and any next steps
You want immediate certainty
You can also proactively order a comprehensive kidney function test that includes both creatinine and cystatin C. This is especially useful if you have a lot of muscle mass and want an accurate baseline, if you have risk factors for kidney problems, if you want to monitor your health annually, or if you’re about to undergo major medical treatment.
How much does a cystatin C test cost?
A Cystatin C test is affordable and well worth the investment for the peace of mind and accurate information it provides. Check our website for current prices and packages.
What to do with the results?
If your cystatin C level is normal, your kidneys are functioning well, and the abnormal eGFR was due to your muscle mass. No further action is needed in that case. If Cystatin C is slightly abnormal, we recommend repeating the test in a few months to see if the level remains stable, possibly with a minor lifestyle adjustment. If cystatin C confirms reduced kidney function, we recommend contacting your primary care physician for further diagnosis and guidance.
Symptoms that may indicate kidney problems
Reduced kidney function often progresses without symptoms for a long time, but there are signs to watch out for: fatigue and difficulty concentrating due to the buildup of waste products, changes in urination such as foamy urine or blood in the urine, fluid retention in the ankles, legs, or eyelids, high blood pressure, nausea and loss of appetite, itching caused by a buildup of waste products, and muscle cramps due to an electrolyte imbalance.
Important: These symptoms can also occur with many other conditions. If you have one or more of these symptoms, contact your primary care physician, even if your blood test results appear normal.
The good news is that most people with an abnormal eGFR due to biological variation do not experience any of these symptoms. That in itself is a reassuring sign that the kidneys are likely functioning normally.
Tips for Healthy Kidney Function
Whether you have perfect kidney function or a slightly reduced eGFR, your kidneys deserve good care. Stay well-hydrated, especially around workout times, as this helps your kidneys perform their filtering function. Exercise regularly to keep your blood pressure healthy. Watch your salt intake, because too much salt raises your blood pressure and puts a strain on your kidneys. Be careful with pain relievers: long-term use of NSAIDs such as ibuprofen can damage your kidneys, so use them only when necessary and as prescribed. Monitor your blood sugar and blood pressure, as diabetes and high blood pressure are the leading causes of kidney damage. Don’t smoke, as smoking impairs blood flow to your kidneys. Eat a varied diet rich in vegetables, fruits, and whole-grain products. Limit your alcohol intake, and have your kidney function checked regularly, especially if you have risk factors, because early detection makes a world of difference.
Be careful with supplements: some supplements can put a strain on your kidneys at high doses. Always discuss supplement use with a healthcare professional, especially if you already have reduced kidney function.
Conclusion: Certainty through the right test
An abnormal eGFR value is no reason to panic. Thanks to cystatin C, we can determine with certainty whether your kidneys are functioning optimally, without biological factors such as muscle mass skewing the results.
At bloodtesting.nl, we believe in proactive healthcare with personalized attention. That’s why we call you if your results are abnormal, explain the possible causes, and recommend the appropriate next steps. Whether that means reassuring an athlete with a lot of muscle mass or a timely referral for further medical care, you’ll receive advice tailored to your situation.
The Cystatin C test is the gold standard for accurately assessing your kidney function. For the price of a single test, you get clarity, peace of mind, and—if necessary—timely action. Your health deserves precision. You deserve certainty. And that’s exactly what Cystatin C offers you.
Ready to get clarity on your kidney function?
Have you recently received an abnormal eGFR result? Order a Cystatin C test right away through our website. Within a few days, you’ll have certainty about your kidney function. If we’ve already called you about your results, you can simply click through to the order page we’ve shared.
Want a complete picture right away? Consider our comprehensive kidney function panel, which includes both creatinine and cystatin C. Ideal for people with a lot of muscle mass, for an annual health checkup, or when you simply want the most accurate baseline for your kidney health.
Questions?
Please feel free to contact us. We’d be happy to help you determine which test is best suited to your situation.
Your kidneys work hard for you every day. Give them the attention they deserve—with the right test, at the right time.
