Cystatin C
Biochemistry Overview / Directory
Purpose of the test
Cystatin C is used as an indicator of renal glomerular function.
It is a low molecular weight, 13 kDa, non-glycosylated basic protein belonging to the cystatin super-family of cysteine protease inhibitors. It is produced by all nucleated cells and is freely filtered at the glomerulus, almost completely reabsorbed, degraded and not secreted by the proximal renal tubular cells. Thus it fits the criteria for a good marker of the glomerular filtration rate (GFR). The plasma concentration of Cystatin C is almost exclusively determined by the GFR.
Measurement of serum creatinine has traditionally been the most commonly used estimate of GFR. It is however influenced by muscle mass, patient age and plasma bilirubin concentration (high bilirubin causes negative interference), making this unsuitable in children with liver disease. Calculated estimations of GFR, the creatinine clearance and the Schwartz formula, are both based on serum creatinine measurements and, therefore have the same drawbacks.
Similarly, the gold standard method of chromium-51 EDTA (51Cr-EDTA) used to estimate GFR is also unsuitable for routine use. This is an invasive and expensive method utilising a dose of radiation, taking several blood samples and requiring a 5-hour day case admission.
Serum cystatin C measurement dose not require radio-isotopes, is non-invasive and not affected by muscle mass or gender. Serum levels reach adult ranges by 1 year of age.
Cystatin C is particularly useful when there is a need to accurately identify a reduced GFR in patients where serum creatinine is of limited use. An example is the monitoring of nephrotoxicity in children receiving immunosuppressant drugs following organ transplantation (in the case of renal transplant, graft rejection could also be monitored).
Reference Range
Up to 3 months 0.55 – 1.71 mg/L
3 months to 50 years 0.55 – 1.00 mg/L
> 50 years 0.61 – 1.40 mg/L
Sample Requirements
0.5 ml serum/plasma (heparin or EDTA). Serum/plasma can be taken from gel separation tubes if necessary.
Storage and Transport
Stable for 7 days at 2 – 8 degrees Celsius. External samples should be sent by first class post or faster.
Turnaround Time
24 hours (72 hours across the weekend)
Price
Price available on application - please contact adrian.turner@kch.nhs.uk. Discounts could be available for significant workloads.
Contacts
Dr Elizabeth Okokon (clinical aspects):
Steve Hodges (laboratory aspects):
T: 020 3299 3501
