The Cystatin C Blood Test
The cystatin C blood test is used to monitor kidney function in people experiencing or suspected to have, kidney disease (1). If a physician suspects that you’re suffering from impaired kidney function they will likely order a cystatin C blood test.
The cystatin C test can be done instead of the creatinine or creatinine clearance test in cases where those tests aren’t appropriate (2), such as:
- In patients with cirrhosis
- Cases of morbid obesity
- Reduced or more than "normal" amount of muscle mass
- Patients taking creatine supplements that are increasing their creatinine and decreasing their estimated creatinine clearance (eGFR)
Since the concentration of cystatin C in the blood will not change due to infection or inflammation and isn’t affected by body weight, lean mass, drugs or diet - cystatin C level could be a more reliable indicator of kidney health than creatinine (2). On the other hand, creatinine has been shown to be affected by higher body weight and lean mass.
Testing for cystatin may also be useful to detect kidney disease early when the affected person isn’t experiencing many (if any) symptoms, and other test results are normal.
What is Cystatin C?
Cystatin C is a protein found in almost all the cells in the body. Cystatin C helps to regulate the activity of enzymes needed to break down bigger molecules both inside of and around those cells (3).
Cystatin C level in the blood can be measured to determine how well the kidneys are functioning (3). The kidneys, liver, and spleen all work to filter the blood and excrete any waste substances as urine. This filtration process happens in a microscopic structure in the kidneys; the glomerulus (4). Each kidney contains approximately one-million glomeruli that function as tiny sieves to extract waste. The glomerular filtration rate (GFR) rate is a measure of how well the glomeruli are filtering blood (4).
If the kidneys and glomeruli are impaired, the GFR could decrease. A decreased GFR can cause certain proteins to accumulate (like cystatin C and creatinine) in the blood.
So, the Cystatin C blood test is used to measure circulating levels of cystatin C. The levels of cystatin C in the blood can be used to assess the glomerular filtration rate to indirectly provide an assessment of kidney function.
Risk factors for Kidney Disease
Over 30 million American adults have kidney disease, but most of them aren’t aware of it (5). This lack of awareness is because people who have kidney disease tend to not experience any symptoms until kidney disease is in its latest stages. This happens when the kidneys are failing or when there are already large amounts of protein present in the urine.
If your risk for kidney problems is already high due to hypertension (high blood pressure, diabetes, kidney failure runs in your family or you’re over 60 years of age (6), it’s crucial that you get tested yearly for kidney disease.
Symptoms that May Mean a Cystatin C Test is Needed
Although the only way to find out if you have kidney disease is to get tested, some symptoms indicate you may be experiencing kidney problems.
These are five symptoms that could indicate impaired kidney function (7):
1) You need to urinate more frequently than normal. If you need to urinate more frequently, particularly at night, it can be a sign of kidney disease. When the kidney’s filters aren’t functioning well, it can result in an increased need to urinate.
2) Swollen ankles and feet. A decrease in kidney function may cause sodium retention which can make feet and ankles swell. Lower extremity swelling can also indicate heart or liver disease and circulatory problems.
3) Foamy urine. The presence of bubbles in your urine, especially persistent ones that require several flushes before they go away, could indicate excess protein is present in the urine. This foam may resemble the foam seen when scrambling eggs, since a protein found in urine is the same protein found in eggs: Albumin.
4) Dry and itchy skin. Healthy kidneys perform many critical functions. From removing waste and excess fluid from the body and helping to make blood cells to helping keep bones healthy by maintaining the correct amount of minerals in the blood. When skin becomes dry and itchy, it can be a symptom of mineral or bone disease that can accompany late-stage kidney disease because the kidneys can no longer maintain the correct balance of minerals in your blood.
5) Fatigue. A severe impairment of kidney function may lead to an excess of toxins and impurities in your blood. This accumulation of toxins can result in fatigue and can make it difficult to concentrate. Tiredness and weakness may also be the result of anemia, which is another complication of kidney disease.
What Else Can Cause High Cystatin C Levels?
While abnormally high cystatin C levels may indicate that you have a potentially serious kidney condition, it’s not the only medical issue that can cause this. Elevated levels of cystatin C could also be caused by conditions like (8,9,10) :
It’s best to speak to your doctor to understand precisely what the results of a cystatin C test mean for your health.
1) Mayo Clinic. Cystatin C with Estimated GFR, Serum. Mayo Clinic Staff. (N.D.) https://www.mayomedicallaboratories.com/test-catalog/Clinical+and+Interpretive/35038
2) Baxmann, Alessandra Calábria et al. “Influence of Muscle Mass and Physical Activity on Serum and Urinary Creatinine and Serum Cystatin C.” Clinical Journal of the American Society of Nephrology : CJASN 3.2 (2008): 348–354. PMC. Web. 25 Nov. 2017.
3) National Kidney Foundation. Cystatin C. NKF Staff. (N.D.). https://www.kidney.org/atoz/content/cystatinC
4) De Martino C., Allen D.J., Accinni L. (1985) Microscopic structure of the kidney. In: Didio L.J.A., Motta P.M. (eds) Basic, Clinical, and Surgical Nephrology. Developments in Nephrology, vol 8. Springer, Boston, MA
5) National Institute of Diabetes and Digestive and Kidney Diseases. Kidney Disease Statistics for the United States. NIDDK Staff. (N.D.). https://www.niddk.nih.gov/health-information/health-statistics/kidney-disease
6) Kazancioğlu, Rumeyza. “Risk Factors for Chronic Kidney Disease: An Update.” Kidney International Supplements 3.4 (2013): 368–371. PMC. Web. 25 Nov. 2017.
7) Mayo Clinic. Chronic Kidney Disease. Mayo Clinic Staff. (N.D.). https://www.mayoclinic.org/diseases-conditions/chronic-kidney-disease/symptoms-causes/syc-20354521
8) INKER, Lesley A et al. “Performance of Creatinine and Cystatin C GFR Estimating Equations in an HIV-Positive Population on Antiretrovirals.” Journal of acquired immune deficiency syndromes (1999) 61.3 (2012): 302–309. PMC. Web. 25 Nov. 2017.
9) Yaffe, Kristine et al. “Higher Levels of Cystatin C Are Associated with Worse Cognitive Function Among Older Adults with Chronic Kidney Disease: The CRIC COG Study.” Journal of the American Geriatrics Society 62.9 (2014): 1623–1629. PMC. Web. 25 Nov. 2017.
10) Madero, Magdalena, and Mark J. Sarnak. “Association of Cystatin C with Adverse Outcomes.” Current opinion in nephrology and hypertension 18.3 (2009): 258–263. Print.