Why Checking eGFR Calculation Method Matters in Your Case
You need to verify how your eGFR was calculated because an eGFR of 70 mL/min/1.73 m² could represent either normal kidney function (if creatinine-based) or mild kidney dysfunction (if cystatin C-based), fundamentally altering your CKD staging and clinical management decisions. 1
The Critical Difference Between Calculation Methods
Your laboratory likely reported only one eGFR value, but there are actually three distinct ways to calculate it:
- eGFRcreat (creatinine-based): Uses only serum creatinine
- eGFRcys (cystatin C-based): Uses only cystatin C
- eGFRcreat-cys (combined): Uses both markers together 2
The combined creatinine-cystatin C equation provides the most accurate estimate, with 94.9% accuracy within 30% of measured GFR, and should be used when both markers are available. 1
Why This Matters for Your Cardiorenal Risk Assessment
While your elevated cystatin C level of 1.04 mg/L does indicate increased cardiovascular risk regardless of the eGFR calculation method 3, 4, knowing which eGFR was reported determines whether you actually have confirmed chronic kidney disease or just elevated cardiovascular risk without kidney disease. 2
The Clinical Implications Are Different:
If your eGFR of 70 was creatinine-based: Your kidneys may actually be functioning worse than reported, since your cystatin C is elevated. KDIGO guidelines recommend measuring cystatin C specifically when creatinine-based eGFR is 45-59 mL/min/1.73 m² to confirm CKD diagnosis 2, 5
If your eGFR of 70 was cystatin C-based: This confirms your kidney function is mildly reduced, and the elevated cystatin C carries both renal and cardiovascular prognostic significance 2, 4
If discordance exists (>20% difference between methods): The combined equation should be used for all clinical decisions, as it demonstrates 84% accuracy versus 50% for creatinine alone in discordant cases 1
What You Should Request From Your Laboratory
The American Journal of Kidney Diseases explicitly recommends that laboratories report both eGFRcys and eGFRcreat-cys whenever cystatin C is measured, to ensure accurate kidney function assessment. 2, 1
You should ask for:
- Your serum creatinine value
- All three eGFR calculations (eGFRcreat, eGFRcys, and eGFRcreat-cys) using the 2012 CKD-EPI equations 1
- Confirmation that the cystatin C assay was traceable to international reference standards 2
The Cardiorenal Connection
Your concern about cardiorenal syndrome is valid. Elevated cystatin C is an independent predictor of cardiac events in heart failure patients, with a hazard ratio of 1.94 per standard deviation increase, even when creatinine levels are normal. 3
Research demonstrates that:
- Patients with cystatin C ≥1.0 mg/L have significantly higher cardiac event rates (38.7% vs 10.3%) compared to those with normal levels 3
- The interplay between kidney and heart function begins prior to symptomatic disease, with higher eGFR associated with better left ventricular function even in asymptomatic individuals 6
- Cystatin C-based eGFR predicts prognosis more accurately than creatinine-based equations in acute heart failure patients 4
Bottom Line for Clinical Decision-Making
Your cystatin C level of 1.04 mg/L places you above the 1.0 mg/L threshold associated with substantially increased cardiovascular risk, but you need the complete eGFR picture to determine if you have confirmed CKD requiring nephrology referral and CKD-specific management. 5, 3
The calculation method matters because:
- Medication dosing adjustments depend on accurate eGFR 2
- Specialist referral decisions require confirmed CKD diagnosis 5
- Risk stratification for cardiovascular events differs based on whether kidney disease is present 2, 4
If your laboratory cannot provide all three eGFR values, request that both creatinine and cystatin C be measured simultaneously with all three calculations reported, as this is the KDIGO-recommended approach for accurate assessment. 1, 5