Cystatin C

 
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Cystatin C is a small (13 kD) protein that is used as a marker of glomerular filtration rate (GFR) or kidney function, particularly in chronic renal disease (CKD). Serum cystatin C is generally considered superior to creatinine as a marker of GFR in dogs (Ghys et al 2014 review) but not in cats (Ghys et al 2016).

Physiology

Cystatin C is produced by all cells in the body at a constant rate and functions as an inhibitor of cysteine proteases. It is normally filtered through the glomerulus (being such a low molecular weight protein) and is resorbed in the proximal convoluted tubules, where it is degraded. Since it is almost completely reabsorbed, urinary concentrations of cystatin C are quite low and high concentrations in urine can be used to indicate proximal renal tubular damage. Extrarenal routes of excretion do exist, particularly when serum concentrations are high secondary to renal disease resulting in a high GFR. As GFR decreases, glomerular filtration of cystatin C decreases, resulting in increased serum concentrations of this protein.

Methodology

Cystatin C is currently not offered by Cornell University. The protein is detected using automated immunologic-based turbodimetric or nephelometric assays. In the standard immunoturbidometric assay (most common assay used in animals since it can be measured with standard chemistry analyzers), turbidity is altered by the formation of antigen-antibody complexes (cystatin C and anti-cystatin C antibodies on beads), which can be detected by spectrophotometry in automated analyzers. Species specific anti-Cystatin C antibodies are not available, hence commercial assays use anti-human cystatin C antibodies, which may demonstrate variable cross-reactivity with animal species. Several studies have shown that commercial assays can be used to detect cystatin C in canine and feline serum and feline urine (Ghys et al 2014 review).

Units of measurement

Cystatin C concentration is measured in mg/L (SI units).

Sample considerations

Sample type

Serum, urine

Stability

There is variability in published reports for stability, however cystatin C is generally stable for up to 2 days at 4ºC and at least a month frozen at -20ºC in humans.

Test interpretation

Studies in dogs to date demonstrate that cystatin C (reciprocal) appears to correlate better with GFR and appears to be a more sensitive marker than creatinine for renal disease. It is still unclear if cystatin C is increased in dogs with prerenal azotemia (more studies are needed in which GFR is measured using other methods, e.g. inulin clearance). Cystatin C concentrations in healthy cats overlap with those in cats with chronic kidney disease, but it is possible that the clinically healthy cats had subclinical renal disease. Increased cystatin C in urine is a potential marker of proximal renal tubular injury but this has not been adequately evaluated in veterinary medicine. Urinary cystatin C concentrations were not increased in cats with chronic renal disease in one study (Ghys et al 2016), however proximal tubular injury may not be occurring n these animals. Unlike creatinine, serum cystatin C concentrations are not influenced by muscle mass. Like creatinine, cystatin C is influenced by feed intake, with decreases occurring for up to 9 hours after ingestion of a meal in dogs; this may be related to a feeding-associated increase in GFR (Ghys et al 2014 review). In contrast, creatinine will increase somewhat after a meal due to increased production from gastrointestinal-derived amino acids. The influence of drugs on cystatin C concentration is currently unknown.

Increased cystatin C concentration in serum

Reported upper and lower limits in healthy dogs and cats (health based mostly on normal creatinine concentrations) are approximately 0.4-1.6 mg/L and o.58-1.59 mg/L, respectively (Ghys et al 2014, Ghys et al 2015).
  • Pathophysiologic: As indicated above, measurement of cystatin C is usually used as a marker of decreased GFR in dogs (Ghys et al 2014 review).
    • Prerenal azotemia: The effect of prerenal azotemia on cystatin C concentrations is yet to be ascertained in animals.
    • Non-renal disease: Cystatin C concentrations may be increased in cats with hyperthyroidism, potentially limiting usefulness of the test (Williams et al 2016). In the latter study, cystatin C concentrations decreased in some cats after they became euthyroid post treatment, however changes in GFR were not evaluated after treatment. Other animals with non-renal disease may have high cystatin C which could be due to non-renal disease or reflect decreased GFR in affected patients.
    • Acute kidney injury (AKI): Insufficient studies have been done to determine if cystatin C is a sensitive marker of AKI.
    • Chronic kidney disease: In general, studies in dogs show that cystatin C concentrations are higher in dogs and cats with CKD than healthy dogs or cats, however many studies use creatinine as the index marker of CKD and there is overlap between the groups.  Due to this overlap, cystatin C is not considered a useful marker of renal injury.

Increased cystatin C concentration in urine

This could be a potential marker of proximal renal tubular injury. Proteinuria can affect results (by inhibiting renal tubular absorption), so measurement of protein concurrently along with creatinine values (and determination of a cystatin C/creatinine ratio) is recommended (Ghys et al 2014 review).  
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