Understanding Measured GFR: Accuracy in Kidney Function Assessment

Infographic mGFR vs eGFR

This blog is my personal summary of several insightful presentations from the ERA25 Congress, held in Vienna, June 4–7, 2025. The event gathered leading nephrology experts to explore the latest developments in how we measure kidney function, especially the ongoing debate between estimated GFR (eGFR) and measured GFR (mGFR).

Understanding how well your kidneys work is essential—whether you're being monitored for chronic kidney disease (CKD), adjusting medications, or being evaluated as a potential kidney donor. Yet not all kidney function tests are created equal. In this article, I compare eGFR (based on creatinine, cystatin C, or both) with mGFR using iohexol clearance with dried blood spots (DBS)—a more precise but less commonly used method. My goal is to help patients and healthcare providers better understand the strengths and limitations of each method so they can make informed decisions based on the most accurate data available.

Key Takeaways

• eGFR (estimated GFR) is convenient and widely used, but can be inaccurate, especially in people with very high or low muscle mass or BMI.

• Measured GFR (mGFR) using iohexol-DBS offers much higher accuracy, especially for drug dosing and high-risk patients.

• The best eGFR formula for accuracy combines creatinine and cystatin C (eGFRcr-cys), but it still falls short compared to mGFR.

• Clinicians must balance practicality with precision—and in certain cases, only a true mGFR will give the confidence needed to make safe decisions.

What Is GFR and Why Does It Matter?

Glomerular Filtration Rate (GFR) measures how well your kidneys filter waste. A higher GFR means better kidney function.

• Estimated GFR (eGFR) is calculated using blood tests like creatinine or cystatin C and is widely used for CKD diagnosis and monitoring.

• Measured GFR (mGFR) directly tracks how a filtration marker like iohexol clears from your blood. It's more accurate, but more complex to perform.

eGFR: Fast but Sometimes Flawed

Most patients have had their kidney function estimated via eGFR, typically using the creatinine-based CKD-EPI formula. While fast and cost-effective, it can misrepresent true kidney function due to external influences like: GFR measurement accuracy kidney function

• Muscle mass

• Age

• Gender

• Inflammation

• Diet

This has led researchers to ask: Can we do better?

Comparing eGFR Formulas: Creatinine, Cystatin C, or Both?

The Three Main Types of eGFR

Formula	Marker Used	Strengths	Weaknesses
eGFRcr	Creatinine	Widely available, inexpensive	Affected by muscle mass, diet, age
eGFRcys	Cystatin C	Less influenced by muscle	More costly, affected by inflammation
eGFRcr-cys	Creatinine + Cystatin C	Most accurate eGFR overall	Requires both tests, higher cost

What the Studies Say

Sehoon Park (UK Biobank, 300,000+ participants)

• Found large differences between eGFRcr and eGFRcys in the same patients.

• eGFRcys predicted long-term kidney outcomes more reliably.

• Developed an adjusted model incorporating variables like age, sex, height, weight, and BMI to better approximate eGFRcys from eGFRcr, which improved the prediction of clinical outcomes.

  
  

  William A. Russell (Karolinska Institute – SCREAM Project)

• Studied patients with extreme BMI (very thin or very obese).

• Found that eGFRcr-cys outperformed both eGFRcr and eGFRcys, especially when indexed to body surface area.

• Non-indexed versions added a modest improvement in some subgroups.

Bottom line: eGFRcr-cys is the most accurate eGFR but still not as precise as mGFR.

mGFR with Iohexol-DBS: The Gold Standard, Evolved

Measured GFR using iohexol-DBS provides a true filtration rate, not an estimate. It involves injecting a contrast agent (iohexol) and measuring how quickly it disappears from the blood via dried blood spot (DBS) sampling.

Advantages:

• Superior accuracy: Over 90% P30 (within 30% of true value), compared to 75–85% for eGFR.

• Less invasive than traditional plasma-based methods.

• Ideal for:

  • Patients with abnormal muscle mass or BMI

  • Children and elderly

  • Nephrotoxic drug dosing

  • Kidney donor evaluation

  • Borderline eGFR cases

 Evidence from Key Studies:

• Schwartz et al. (2021): Found iohexol-DBS GFR closely matched plasma GFR (R² > 0.95) in children.

• Delanaye et al. (2016): Showed iohexol-mGFR was more accurate than both eGFRcr and eGFRcys across all CKD stages.

• Luis-Lima et al. (2017): In 2,903 patients, only 30% of eGFR results were accurate within 10%. Measured GFR using iohexol-DBS had a Concordance Coefficient of 0.996 and Total Deviation Index <10%, showing remarkable precision (90% of values were within ±9.5% of the reference mGFR).

Why Does Accuracy Matter?

Inaccurate kidney function measurements can lead to:

• CKD misclassification (up to 1 in 3 patients)

• Incorrect drug dosing, especially for toxic meds

• Risky kidney donations

• Confusing or delayed diagnoses

If your muscle mass, weight, age, or other conditions make standard eGFR less reliable, measured GFR could be the safer option.

Summary Table: eGFR vs mGFR (Iohexol-DBS)

Aspect	eGFR (Creatinine/Cystatin/Combined)	mGFR (Iohexol-DBS)
Method	Calculated from blood markers + formulas	Direct measurement
Accuracy	75–85% P30 (lower in high/low BMI)	>90% P30
Ease of Use	Simple, low-cost, widely available	Less invasive with DBS, but costlier
Best For	Routine screening	Precision medicine, drug dosing, CKD staging in special populations
Limitations	Influenced by body composition, diet	Requires iohexol injection, lab handling

Final Thoughts: Is It Time to Consider Measured GFR?

Estimated GFR (eGFR) remains the standard tool in everyday clinical practice—it's fast, affordable, and widely accessible. But it's not always accurate enough, particularly for people at the edges of the bell curve.

You or your clinician may want to consider measured GFR (mGFR) using iohexol-DBS if:

• Your eGFR fluctuates or seems inconsistent

• You’re at the extremes of body size (underweight or obese)

• You take medications that require precise kidney function for safe dosing

• You’re being evaluated as a kidney donor

• You have early-stage CKD or borderline kidney function

While eGFR is convenient, its limitations can lead to misdiagnoses or suboptimal treatment in certain groups. Measured GFR with iohexol-DBS offers a highly accurate and minimally invasive alternative, and may become more widely used as awareness and accessibility increase.

If you're facing important decisions about your kidney health, especially in situations where accuracy matters most, talk to your nephrologist about whether mGFR is right for you.

Not Sure Where to Start? I Can Help

If you're feeling overwhelmed about what to eat, how to support your kidney health, or what steps to take next—you're not alone. I offer personalized nutritional therapy for people with chronic conditions, including those navigating CKD. Together, we can build a clear plan based on science, your lab values, and your individual needs.

Book a consultation or get in touch to take the next step toward clarity and confidence in your kidney and health care.