Cystatin C test explained

Cystatin C is a small protein that almost every cell in the body produces at a steady rate. It is freely filtered through the glomeruli of the kidneys and then almost completely reabsorbed and broken down in the kidney tubules, so its concentration in the blood depends mostly on how well the kidneys are working. When the filtering function of the kidneys declines, the amount of cystatin C in the blood rises. According to MedlinePlus, the cystatin C level is used as an alternative or an addition to creatinine when calculating the estimated glomerular filtration rate (eGFR) — the main indicator of kidney function. Unlike creatinine, whose level is affected by muscle mass, age, sex and diet (for example how much meat is eaten), these factors have practically no effect on the concentration of cystatin C. That is why some researchers consider it a more accurate marker of kidney function in certain groups — for example people with a smaller or larger muscle mass, seriously ill patients or older patients.

• When kidney function needs to be clarified and the creatinine-based eGFR may be inaccurate (for example because of unusual muscle mass, an amputation or serious illness)
• When a general creatinine test shows a borderline or unclear result and additional confirmation is wanted
• In chronic kidney disease — to follow the change in function over time
• When the risk of kidney disease is higher — with diabetes, high blood pressure, or kidney disease in the family
• When assessing medicine doses, where clearance of the medicine depends on kidney function

The content of the sources cited on this page (MedlinePlus, NHS) does not give a specific numeric reference limit in mg/l — ranges depend on the method the laboratory uses, so you will find the exact interval on your own laboratory's report.

A raised cystatin C

A raised amount of cystatin C in the blood indicates a reduced filtering function of the kidneys — according to MedlinePlus, eGFR is calculated from it, and a low eGFR means possible kidney disease, while a very low one means possible kidney failure. The risk factors that can reduce function are diabetes, high blood pressure, heart disease, a family history of kidney disease, being older than 50, smoking and obesity.

Possible causes

• High blood pressure — it damages the blood vessels of the kidneys
• Diabetes — excess glucose damages the filters of the kidneys
• Kidney infections and glomerulonephritis (inflammation of the kidneys), inherited polycystic kidney disease
• Obstruction to the flow of urine — kidney stones, an enlarged prostate
• Long-term use of some medicines (for example lithium preparations or non-steroidal painkillers)

Possible symptoms

• In the early stages kidney disease usually causes no symptoms; at a more advanced stage the symptoms listed below can appear
• Tiredness
• Swollen ankles, feet or hands
• Shortness of breath
• Feeling unwell, nausea
• Blood in the urine

What to discuss with a doctor

• Creatinine and eGFR — to compare both indicators of kidney function
• Blood pressure and glucose readings — a common cause of reduced kidney function
• A urine test (the albumin-to-creatinine ratio) — to assess signs of kidney damage
• A review of the medicines you take with your doctor, if some of them can affect the kidneys
• When to repeat the test, so that the change over time can be seen

Related tests

• Creatinine
• GFR (eGFR)
• Urea
• Uric acid
• Albumin-to-creatinine ratio
• Creatinine clearance