Glomerular Filtration Rate (GFR) measures the rate water and dissolved substances are filtered out of the blood per unit time. Hence GFR indicates level of kidney function.
Cystatin C is effectively filtered by the glomerulus and fully reabsorbed and broken down by the renal tubules for recycling. A raised level of serum or plasma cystatin C correlates closely with the GFR. Cystatin C therefore acts as an excellent indicator of reduced kidney function. The relationship between cystatin C level and GFR is valid over the whole range of kidney function observed in the human body [1].
The cystatin C concentration in serum/plasma is uninfluenced by age, gender and muscle mass. For this reason, several studies claim that cystatin C is a better GFR marker than for example creatinine, especially within in the creatinine-blind range [2-5].
Serum creatinine values are not a direct measure of GFR. Creatinine values are dependent on variables including age and body mass, which must be corrected for when calculating the GFR. Importantly, GFR can be reduced by as much as 50% while the serum creatinine value is still within the normal range – this is what is called the “creatinine-blind area” that is visualised in the Gentian Cystatin C ruler below.
NKF-KDIGO Guideline; GFR Categories in Chronic Kidney Disease [6,7]
Normal | Healthy kidneys - normal GFR | GFR > 90 mL/min/1.73m2 |
G1 | Kidney damage with normal or elevated GFR | GFR > 90 mL/min/1.73m2 |
G2 1 | Kidney damage and mild decrease in GFR 2 | GFR of 60 - 89 mL/min/1.73m2 |
G3a | Mild to moderate decrease in GFR | GFR of 45 – 59 mL/min/1.73m2 |
G3b | Moderate to severe decrease in GFR | GFR 30 – 44 mL/min/1.73m2 |
G4 | Severe decrease in GFR | GFR 15 – 29 mL/min/1.73m2 |
G5 | Kidney failure - End Stage Renal Disease (ESRD) | GFR < 15 mL/min/1.73m2 |
1In the absence of evidence of kidney damage, neither Stage 1 nor Stage 2 fulfill the criteria for Chronic Kidney Disease, CDK
2 Relative to young adult level
The Gentian Cystatin C ruler shows the relationship between GFR and the level of serum cystatin C. It also shows the creatinine blind area the stages of kidney disease as classified by the NKF-KDIGO Guideline:
The ruler shows the range for:
The Gentian Cystatin C Immunoassay is FDA510(k) cleared and CE-marked. The Gentian Cystatin C Immunoassay is a Particle-Enhanced Turbidimetric Immunoassay (PETIA) for in vitro diagnostic testing of cystatin C in human plasma and serum samples. The Gentian Cystatin C Immunoassay is rapidly performed in only 10 minutes and can be applied on a wide range of clinical chemical analysers with documented high inter instrument accuracy [8].