From the Guidelines
The discrepancy between the creatinine-derived eGFR of 89 and cystatin C-derived GFR of 57 likely indicates that the true GFR is closer to the cystatin C value of 57. This difference occurs because creatinine-based estimates can overestimate kidney function in certain patient populations, such as those with reduced muscle mass, vegetarians, the elderly, or those with liver disease, as creatinine is affected by these factors 1. Cystatin C, on the other hand, is generally considered more accurate as it's less influenced by non-renal factors like muscle mass, diet, medications, and age, making it a better marker for estimating GFR, especially in patients with subtle changes in kidney function 1. Some key points to consider include:
- Cystatin C is produced at a constant rate and is not secreted, making it a more reliable marker than creatinine 1
- Cystatin C is reabsorbed by tubule epithelial cells but subsequently catabolized, so it does not return to the blood flow, which reduces variability in measurements 1
- Studies have shown cystatin C to be an accurate marker of subtle changes in GFR and diagnostically superior to creatinine with a significantly better correlation with GFR 1 In clinical practice, when there's a significant discrepancy like this, the lower value (cystatin C-GFR) is typically more reliable, especially in patients with muscle wasting, malnutrition, liver disease, or advanced age. For this patient, I would recommend using the cystatin C-derived GFR of 57 for clinical decision-making, including medication dosing and risk assessment, as it would classify the patient as having moderate kidney dysfunction (Stage 3a CKD) rather than near-normal function, which has important implications for clinical management and monitoring. Key considerations for clinical management include:
- Medication dosing adjustments to prevent toxicity
- Increased monitoring for signs of kidney dysfunction
- Lifestyle modifications to slow disease progression
- Regular follow-up to assess disease progression and adjust management plans as needed.
From the Research
Reasons for the Difference
- The difference between creatinine-derived eGFR and cystatin C-derived GFR can be due to various factors, including muscle mass and nutritional status, which have differential effects on eGFR based on cystatin C and creatinine 2.
- Cystatin C is a better marker of GFR in patients with heart failure, as it is less affected by muscle mass and nutritional status 2.
- The association between risk factors and GFR was more accurate by eGFR based on creatinine alone, as cystatin C is associated with non-GFR determinants such as obesity, albuminuria, hypertension, diabetes, higher triglycerides, and higher C-reactive protein 3.
Which Value is Closer to the True One
- The combined creatinine-cystatin C equation performed better than equations based on either of these markers alone and may be useful as a confirmatory test for chronic kidney disease 4.
- In patients with chronic kidney disease, a large positive difference between eGFRcreat and eGFRcys was associated with a higher risk of major adverse cardiovascular events (MACE) and faster coronary artery calcification (CAC) progression 5.
- More negative values of eGFRdiff (i.e., eGFRScr > eGFRcysC) were associated with worse clinical outcomes, including mortality, poor quality of life, and frailty 2, 6.
Clinical Implications
- Clinicians should be aware of the potential differences between eGFR based on creatinine and cystatin C and consider using both markers to estimate GFR, especially in patients with heart failure or chronic kidney disease 2, 6.
- A large difference between eGFRcreat and eGFRcys may indicate a higher risk of adverse outcomes, and patients with such a difference should be closely monitored for cardiovascular disease and other complications 2, 6, 5.