Facts about cystatin C:
(1) A non-glycosylated basic protein with molecular weight of 13.4 kD.
(2) Member of cystatin superfamily of cysteine protease inhibitors.
(3) It is produced at a constant rate by nucleated cells.
(4) The serum concentration is mainly determined by the glomerular filtration rate. It is freely filtered in the renal glomeruli, with resorption and catabolism in the proximal renal tubules.
Benefits over creatinine:
(1) Serum cystatin starts to increase when GFR is 88 mL/min per 1.73 square meter BSA, while serum creatinine increases when the GFR is 75 mL/min per 1.73 square meter BSA.
(2) Serum cystatin is more sensitive for declining GFR.
(3) It is a better marker for GFR in preterm infants.
(4) Serum levels between men and women are similar, although a few authors indicate that it may be lower in women.
Reference range for cystatin C:
(1) The reference range for plasma cystatin C is stable from 1 to 50 years of age.
(1a) Finney et al: 0.51 to 0.98 mg/L using the PENIA method
(1b) Randers et al: 0.54 to 1.21 mg/L using the PETIA method
(2) Under 1 year of age the plasma cystatin C levels are higher, reflecting the immaturity of the kidneys.
(2a) Randers et al (1999, nephelometry): upper limit of reference range 1.89 mg/L at < 1 month, 1.17 for 1-12 months, and 0.95 for > 12 months.
(3) Serum values may rise over age 70 years (Price et al, page 63).
Equation of Randers et al (1998, 1999)
1 / (serum cystatin in mg/L) =
= (0.0062 * (clearance in mL per min per 1.73 BSA)) + 0.2736
Rewritten:
glomerular filtration rate in mL per min per 1.73 BSA =
= (161.29 / (serum cystatin in mg/L)) – 44.13
where:
• A cystatin value of 1.22 would correspond to a creatinine clearance of 88 mL/min per 1.73 square meters, which is a higher value than those reported by other authors.
Equations of Orlando et al (2002)
1 / (serum cystatin in mg/L) =
= (0.009 * (clearance in mL per min per 1.73 BSA)) + 0.22
Rewritten:
glomerular filtration rate in mL per min per 1.73 BSA =
= (111.11 / (serum cystatin in mg/L)) – 24.444
where:
• A cystatin value of 1.01 would correspond with a creatinine clearance of 88 mL/min per 1.73 square meters.
In patients with cirrhosis:
1 / (serum cystatin in mg/L) =
= (0.007 * (clearance in mL per min per 1.73 BSA)) + 0.27
Rewritten:
glomerular filtration rate in mL per min per 1.73 BSA =
= (142.857 / (serum cystatin in mg/L)) – 38.57
Figure 2 in Coll et al (2000)
In Figure 2 in Coll et al, cystatin is related to GFR as:
glomerular filtration rate in mL/min per 1.73 square meter =
= (97.84 / (serum cystatin in mg/L)) –12.6
where:
• A cystatin of 0.98 would correspond to a creatinine clearance of 88 mL/min per 1.73 square meters.
