From the Guidelines
If your eGFR is higher when calculated using cystatin C compared to creatinine, it suggests that your kidney function may be better than what the creatinine-based estimate indicates, and your healthcare provider may consider using an average of both estimates or relying more on the cystatin C-based value for medication dosing and disease management. This discrepancy often occurs in individuals with reduced muscle mass, such as elderly people, those with chronic illness, or individuals with malnutrition, because creatinine levels are influenced by muscle mass and can underestimate kidney function in these populations 1. Cystatin C is produced at a constant rate by all nucleated cells and is less affected by muscle mass, diet, or gender, making it potentially more accurate for certain individuals. However, cystatin C levels can be elevated in conditions like thyroid dysfunction, inflammation, or obesity, which might affect the eGFR calculation.
Key Considerations
- The difference between the two measurements is important because accurate kidney function assessment affects medication dosing, as highlighted in the KDIGO 2024 clinical practice guideline 1, which recommends considering GFR when dosing medications cleared by the kidneys.
- The guideline also suggests that for most people and clinical settings, validated eGFR equations using SCr are appropriate for drug dosing, but where more accuracy is required, use of equations that combine both creatinine and cystatin C, or measured GFR may be indicated 1.
- It is essential to discuss the results with your healthcare provider, who can consider your specific clinical situation and make informed decisions about your treatment plan, taking into account the potential effects of non-GFR determinants of the filtration markers, or volume of distribution that are not in a steady state 1.
Clinical Implications
- Understanding the difference between eGFR calculations using cystatin C and creatinine can help healthcare providers make more accurate decisions about medication dosing and disease management.
- The use of cystatin C-based eGFR estimates may be particularly important in individuals with reduced muscle mass or other conditions that can affect creatinine levels.
- Healthcare providers should consider the potential limitations and biases of each estimation method when interpreting eGFR results and making clinical decisions 1.
From the Research
Differences in eGFR Calculations
- The estimated Glomerular Filtration Rate (eGFR) calculated using Cystatin C can differ from the eGFR calculated using Creatinine 2, 3, 4, 5.
- A higher eGFR when calculated with Cystatin C than with Creatinine may indicate differences in the non-GFR determinants of Cystatin C, such as muscle mass, inflammation, and obesity 2.
- Studies have shown that Cystatin C-based eGFR (eGFRcys) can provide a more accurate estimate of kidney function than Creatinine-based eGFR (eGFRcr) in certain clinical subpopulations, particularly in patients with muscle wasting diseases or those who are malnourished 3, 4, 5.
Clinical Implications
- The difference between eGFRcr and eGFRcys can have significant clinical implications, as it may affect the diagnosis and management of kidney disease 3, 4.
- Patients with a lower eGFRcys than eGFRcr may be at a higher risk of adverse outcomes, including kidney failure, cardiovascular disease, and mortality 4.
- The use of Cystatin C in combination with Creatinine can provide a more accurate estimate of kidney function and improve clinical decision-making 3, 6.
Limitations and Future Directions
- Further research is needed to understand the performance of Cystatin C-based eGFR in diverse clinical populations and to achieve assay standardization 6.
- The availability of Cystatin C testing needs to be broadened, and partnerships between clinical stakeholders and laboratories need to be developed to promote the clinical use of Cystatin C 6.