From the Guidelines
The normal cystatin C level for a patient with a solitary kidney is between 0.51 and 0.98 mg/L, as the remaining kidney compensates by increasing its filtration capacity. This range is based on the fact that cystatin C is a low molecular weight protein that is almost completely filtered by the glomerulus, and its levels are not affected by muscle mass, age, or gender 1. Even with a single kidney, the body maintains normal cystatin C levels due to the compensatory hypertrophy of the remaining kidney, which increases its filtration capacity to approximately 70-80% of what two kidneys would provide.
Key Points to Consider
- Cystatin C is a protein produced at a constant rate by all nucleated cells and is freely filtered by the glomeruli, making it an excellent marker of kidney function regardless of kidney number 1.
- Unlike creatinine, cystatin C is less affected by muscle mass, age, gender, or diet, providing a more reliable indication of glomerular filtration rate (GFR) 1.
- If a patient with one kidney has a cystatin C level that rises above the normal range, this suggests declining kidney function that should be evaluated by a healthcare provider.
- Regular monitoring of cystatin C levels is particularly important for individuals with a single kidney to detect any early changes in kidney function.
Clinical Implications
- The use of cystatin C as a marker of kidney function is supported by its high sensitivity and specificity in predicting GFR <80 mL/min/1.73 m², with values of 91% and 81%, respectively, in a pediatric cohort 1.
- Cystatin C levels can be used to inform the severity of renal injury in children with acute renal injury, as part of the pediatric modified RIFLE criteria 1.
From the Research
Normal Cystatin C Levels
- The normal cystatin C level in a patient with a solitary kidney (single kidney) is not directly stated in the provided studies.
- However, according to the study 2, the non-parametric reference interval for serum cystatin C was calculated to be 0.51-1.02 mg/l (median: 0.79 mg/l; range: 0.33 - 1.07 mg/l) in patients with normal to moderately impaired kidney function.
Factors Affecting Cystatin C Levels
- Cystatin C levels are mainly dependent on glomerular filtration rate (GFR) 2.
- Age is also a factor that affects cystatin C levels, with a strong, non-linear association between age and cystatin C concentration 3.
- The study 4 found that cystatin C was positively correlated with creatinine serum levels, estimated glomerular filtration rate, and PTH levels.
Comparison with Creatinine
- Cystatin C appears to be a better parameter of GFR than serum creatinine in adults with various types of kidney disease with normal to moderately impaired kidney function 2.
- The study 5 found that cystatin C is more sensitive than serum creatinine to detect early renal dysfunction, especially in patients with eGFR ≥ 60 ml/min/1.73 m2.
Cystatin C in Chronic Kidney Disease (CKD)
- Cystatin C is closely related to the degree of renal dysfunction in non-diabetic patients with CKD 4.
- The study 5 found that both serum creatinine and serum cystatin C were significantly increased across CKD groups, but cystatin C is a better predictor of CKD than creatinine in stages with eGFR ≥ 60 ml/min/1.73 m2.