A patient has a cystatin C of 1.23 mg/L corresponding to an estimated glomerular filtration rate of approximately 60 mL/min/1.73 m²; what are the next steps in management?

Confirm CKD diagnosis with serum creatinine-based eGFR and check for albuminuria

Your cystatin C-based eGFR of approximately 60 mL/min/1.73 m² places you at the critical threshold where you need confirmatory testing with serum creatinine and assessment for kidney damage markers before diagnosing chronic kidney disease (CKD) 1, 2.

Immediate Next Steps

1. Obtain serum creatinine and calculate creatinine-based eGFR (eGFRcr)

The 2021 KDIGO guidelines emphasize that accurate CKD diagnosis requires both creatinine and cystatin C measurements 2. Your cystatin C result alone is insufficient for diagnosis. Calculate eGFRcr using the CKD-EPI creatinine equation 1.

• If eGFRcr is also <60 mL/min/1.73 m²: CKD is confirmed 1, 3
• If eGFRcr is ≥60 mL/min/1.73 m²: CKD is not confirmed by GFR criteria alone 1, 3

This approach is critical because approximately 42% of patients with eGFRcr 45-59 mL/min/1.73 m² are reclassified to eGFR ≥60 when using cystatin C 2. The reverse also occurs—you may have normal creatinine but reduced kidney function confirmed by cystatin C.

2. Check for markers of kidney damage

Even if both eGFR values are ≥60 mL/min/1.73 m², you could still have CKD if kidney damage is present 4. Measure:

• Urine albumin-to-creatinine ratio (UACR) on a spot morning urine sample 1, 2
  • UACR ≥30 mg/g indicates kidney damage and confirms CKD regardless of eGFR 4, 1
  • This must be repeated to confirm persistence (≥3 months) 4

The dual assessment of eGFR and albuminuria is mandatory for proper CKD screening and risk stratification, as both independently predict cardiovascular events, kidney failure, and mortality 2.

Interpreting Discordant Results

Common pitfall: Cystatin C and creatinine can disagree substantially. When eGFRcys differs from eGFRcr, the combined equation (eGFRcr-cys) provides the most accurate estimate 2, 5.

• If you have low muscle mass, malnutrition, liver disease, or significant weight loss, creatinine-based estimates will overestimate your true GFR 6
• In this scenario, cystatin C-based estimates are more accurate 6, 5
• The combined creatinine-cystatin C equation (CKD-EPI 2012) shows the most consistent performance across different patient populations 5, 7

Confirming CKD Diagnosis

CKD requires either of the following persisting for ≥3 months 4:

1. eGFR <60 mL/min/1.73 m² (confirmed by both creatinine and cystatin C ideally)
2. Markers of kidney damage (albuminuria, abnormal imaging, abnormal urine sediment) regardless of eGFR

Repeat testing in 3 months to confirm persistence before making a definitive CKD diagnosis 4, 1.

Risk Stratification After Diagnosis

If CKD is confirmed, your risk for adverse outcomes depends on both your eGFR category and albuminuria category 4, 2:

• eGFR 45-59 mL/min/1.73 m² (Stage G3a) with normal albuminuria = moderate risk
• eGFR 45-59 mL/min/1.73 m² with UACR ≥30 mg/g = high to very high risk
• Lower eGFR or higher albuminuria = progressively higher risk for cardiovascular disease, kidney failure, and death 2

Monitoring Frequency

Once CKD is confirmed 3:

• Assess eGFR and UACR at least annually
• More frequent monitoring (every 3-6 months) if you have higher-risk features: eGFR <45, albuminuria ≥300 mg/g, rapid GFR decline, or progressive albuminuria

Special Considerations

The 2024 KDIGO guidelines now recommend measuring both creatinine and cystatin C in parallel for initial CKD diagnosis and staging, rather than using cystatin C only as a confirmatory test 8, 9. This "triple marker" approach (creatinine, cystatin C, and UACR) provides the most accurate diagnosis and risk stratification 2.

However, after CKD diagnosis is established, routine monitoring can rely on creatinine-based eGFR alone—cystatin C is not required for ongoing surveillance 2.