When should cystatin C be measured?

When Should Cystatin C Be Measured?

Measure cystatin C when creatinine-based eGFR (eGFRcr) is expected to be inaccurate and the GFR assessment will impact clinical decisions such as CKD diagnosis, staging, or drug dosing. 1

Clinical Algorithm for Cystatin C Measurement

The KDIGO 2024 guidelines provide a clear stepwise approach 1:

Step 1: Start with Creatinine-Based eGFR

• Always begin with serum creatinine and calculate eGFRcr as your initial assessment
• This is the standard first-line test for kidney function evaluation

Step 2: Determine if eGFRcr is Likely Inaccurate

Measure cystatin C in these specific situations:

Altered Muscle Mass Conditions

• Extremes of body composition: Very low or very high muscle mass
• Sarcopenia or cachexia: Elderly, malnourished patients, cancer patients
• Bodybuilders or athletes: Increased muscle mass
• Amputees: Reduced muscle mass 1

Dietary Factors Affecting Creatinine

• Low-protein diets
• Ketogenic diets
• Vegetarian/vegan diets
• High-protein diets or creatine supplementation 1

Chronic Illness States

• Malnutrition
• Active cancer
• Heart failure
• Cirrhosis
• These conditions affect both creatinine and cystatin C non-GFR determinants 1

Critical Clinical Decision Points

• Drug dosing decisions where precise GFR is essential (chemotherapy, immunosuppressants)
• CKD diagnosis or staging when eGFRcr is borderline or uncertain
• Kidney donor evaluation
• When eGFRcr conflicts with clinical presentation 1, 2

Which Cystatin C Equation to Use

Once you measure cystatin C, the choice of equation matters:

• Use eGFRcr-cys (combined creatinine-cystatin C equation) in most situations—this is more accurate than either marker alone 1, 3
• Use eGFRcys (cystatin C only) in otherwise healthy individuals with isolated muscle mass abnormalities and no comorbid illness 1
• In patients with comorbidities, always prefer eGFRcr-cys over eGFRcys alone 1

Important Caveats and Limitations

When Cystatin C May Also Be Inaccurate

Be aware that cystatin C has its own non-GFR determinants 1:

• Very high inflammation or sepsis (cystatin C increases)
• High catabolic states
• Exogenous corticosteroid use (increases cystatin C)
• Thyroid dysfunction
• Very low muscle mass (affects both markers)

In these situations, if even more accurate GFR assessment is needed, proceed to measured GFR using exogenous filtration markers (iohexol, iothalamate clearance) 1.

Practical Implementation

The 2024 KDIGO recommendation is clear: Use eGFRcr-cys when eGFRcr is less accurate AND the GFR value will change your clinical management (1C recommendation) 1. This is not a screening test for everyone—it's a confirmatory test when creatinine-based estimates are unreliable.

Research supports that eGFRcr-cys consistently outperforms creatinine-alone equations across diverse populations and provides better prognostic value for outcomes including kidney failure, cardiovascular disease, and mortality 3, 4. The combined equation is particularly valuable when there are large discrepancies between eGFRcr and clinical suspicion 4.

Common Pitfall to Avoid

Don't reflexively order cystatin C on every patient. It adds cost and complexity without benefit when eGFRcr is already accurate. Reserve it for situations where creatinine is misleading due to the specific factors outlined above 2, 5.