What is the appropriate management for an adult patient with an eGFR indicating Stage 3a chronic kidney disease but a lab report stating Stage 1?

Resolving the Discrepancy Between Lab Report and eGFR

When your lab report states Stage 1 CKD but your eGFR indicates Stage 3a (45-59 mL/min/1.73 m²), you should measure serum cystatin C and use the combined creatinine-cystatin C equation (eGFRcr-cys) to determine your true CKD stage, as this combined equation is significantly more accurate than creatinine alone and will resolve the discrepancy. 1, 2

Understanding the Discrepancy

Your situation represents a common clinical scenario where creatinine-based eGFR may be inaccurate due to non-GFR factors:

• Creatinine-based eGFR can misclassify up to 23% of patients when used alone, particularly in the 45-59 mL/min/1.73 m² range 1
• The lab may be reporting "Stage 1" based on other criteria (such as normal albuminuria or imaging findings) while your eGFR clearly indicates reduced kidney function 1
• Low muscle mass, malnutrition, vegetarian diet, or certain medications can cause creatinine to underestimate true kidney function, making eGFR appear worse than it actually is 1, 2

The KDIGO-Recommended Solution

The 2024 KDIGO guidelines provide a clear algorithmic approach for exactly this situation:

Step 1: Measure Cystatin C

• Request a serum cystatin C measurement from your physician 1, 2
• This is specifically recommended when eGFRcr is 45-59 mL/min/1.73 m² without other markers of kidney damage 1, 2
• Cystatin C is independent of muscle mass, age, sex, and diet, making it more reliable than creatinine alone 2

Step 2: Calculate Combined eGFRcr-cys

• The combined creatinine-cystatin C equation (eGFRcr-cys) is 62% accurate compared to only 38% for creatinine alone 2
• This combined equation consistently outperforms either marker alone across all patient populations 1, 2
• The combined value should be used for all clinical decisions, including CKD staging and medication dosing 2

Step 3: Interpret the Results

If eGFRcr-cys ≥60 mL/min/1.73 m²:

• Your lab report is correct—you do not have CKD based on reduced GFR alone 2
• The low creatinine-based eGFR was due to non-GFR factors (likely low muscle mass) 2

If eGFRcr-cys 45-59 mL/min/1.73 m²:

• You have confirmed Stage 3a CKD 2
• The combined equation provides your true kidney function 1, 2

If eGFRcr-cys <45 mL/min/1.73 m²:

• You have Stage 3b or worse CKD 1
• More aggressive management and nephrology referral may be needed 1

Confirm the Diagnosis

CKD requires persistence of abnormalities for ≥3 months:

• Repeat both creatinine and cystatin C measurements in 3 months 2
• Recalculate eGFRcr-cys to confirm stability 2
• A single abnormal measurement does not establish CKD diagnosis 1, 3

Additional Evaluation Needed

While resolving the eGFR discrepancy:

• Check for albuminuria (urine albumin-to-creatinine ratio) if not already done 1, 4
• Albuminuria ≥30 mg/g confirms kidney damage even if eGFR is normal 1
• This would support a CKD diagnosis regardless of the final eGFR value 1

Common Pitfalls to Avoid

• Do not rely on creatinine-based eGFR alone when there is clinical suspicion of inaccuracy 1
• Do not ignore the discrepancy—using a single eGFR measurement can overestimate CKD prevalence by 25% 3
• Be aware that thyroid dysfunction, high inflammation, or exogenous steroid use can affect cystatin C levels and should be considered when interpreting results 2
• Ensure your laboratory uses assays calibrated to international reference standards for accurate cystatin C measurement 2

Clinical Implications

Once you have the combined eGFRcr-cys result:

• Use this value for all medication dosing decisions, not the creatinine-based estimate 2
• If confirmed Stage 3a CKD (eGFRcr-cys 45-59 mL/min/1.73 m²), you will need appropriate CKD management including cardiovascular risk reduction, blood pressure control, and monitoring for complications 4, 5
• Stage 3a CKD is typically asymptomatic, so lack of symptoms does not rule out the diagnosis 6