What to do if eGFR is 70 but unclear if based on creatinine or cystatin C (Cystatin C) with a cystatin C level of 1.04?

Understanding Your eGFR Result: Clarifying the Calculation Method

When you receive an eGFR of 70 mL/min/1.73 m² with a cystatin C level of 1.04 mg/L, you should immediately contact the laboratory or ordering provider to clarify which equation was used, as this fundamentally changes the clinical interpretation and determines whether you have chronic kidney disease.

Why This Matters Clinically

The distinction between creatinine-based and cystatin C-based eGFR is critical because:

• Your cystatin C level of 1.04 mg/L sits right at the upper limit of normal (reference range 0.47-1.03 mg/L), suggesting borderline kidney function 1
• The 2012 CKD-EPI cystatin C equation uses 0.8 mg/L as its reference point, meaning your value of 1.04 would generate an eGFR below the normal threshold 2
• An eGFR of 70 could represent either normal kidney function (if creatinine-based) or mild reduction (if cystatin C-based), fundamentally altering your CKD staging 3

Immediate Action Steps

Step 1: Contact the Laboratory

• Call the laboratory that performed the test and ask specifically: "Was the eGFR of 70 calculated using the creatinine-based equation, the cystatin C-based equation, or the combined creatinine-cystatin C equation?" 3
• Request both values if available: KDIGO guidelines recommend reporting both eGFRcys and eGFRcreat-cys whenever cystatin C is measured 3

Step 2: Obtain Your Serum Creatinine Value

• If you don't already have it, request your serum creatinine level from the same blood draw 3
• This allows calculation of both equations and reveals whether there is discordance between the two methods 4, 5

Understanding Potential Scenarios

Scenario A: eGFR 70 is Creatinine-Based

• If the eGFR 70 was calculated from creatinine alone, your cystatin C of 1.04 mg/L suggests your true kidney function may be lower than 70 6, 7
• This represents a negative eGFRdiff (cystatin C-based eGFR lower than creatinine-based), which occurs in 65% of patients and is associated with higher risk of adverse outcomes 7
• KDIGO guidelines specifically recommend measuring cystatin C in adults with creatinine-based eGFR 45-59 mL/min/1.73 m² to confirm CKD, though your value of 70 falls outside this range 3

Scenario B: eGFR 70 is Cystatin C-Based

• If the eGFR 70 was calculated from cystatin C alone with your level of 1.04 mg/L, this would be internally consistent 2
• You should also obtain a creatinine-based eGFR to determine if there is concordance or discordance 3
• The combined creatinine-cystatin C equation (eGFRcreat-cys) provides the most accurate estimate with 94.9% accuracy within 30% of measured GFR 8, 4, 5

Clinical Significance of Discordance

When eGFR values differ by more than 20% between methods, the combined equation is substantially more accurate than either alone:

• In cases where eGFRcys is lower than eGFRcreat (which your cystatin C level suggests may be the case), the creatinine-based estimate typically overestimates true kidney function by a median of 15 mL/min/1.73 m², while the cystatin C-based estimate underestimates by 8.5 mL/min/1.73 m² 5
• The combined equation (eGFRcreat-cys) has a median bias of only 0.8 mL/min/1.73 m² in these discordant cases 5
• Patients with large negative eGFRdiff have 2.6 times higher risk of acute kidney injury and death compared to those with concordant values 7

Recommended Next Steps

For Your Healthcare Provider

1. Order both serum creatinine and cystatin C simultaneously if not already done 3
2. Calculate all three eGFR values: eGFRcreat, eGFRcys, and eGFRcreat-cys using the 2012 CKD-EPI equations 3, 2
3. If discordance exists (>20% difference), use the combined equation (eGFRcreat-cys) for clinical decision-making as it demonstrates 84% accuracy (P30) versus 50% for creatinine alone and 73% for cystatin C alone in discordant cases 5
4. Assess for factors affecting creatinine accuracy: variable muscle mass, extremes of body composition, malnutrition, or conditions like amputations or spinal cord injury 2, 8

Common Pitfall to Avoid

Do not assume the eGFR is creatinine-based simply because it's more commonly ordered - many laboratories now routinely report cystatin C-based eGFR when cystatin C is measured, but labeling practices vary 3, 9