Cystatin C

What is Cystatin C?

Cystatin C is a small protein produced by virtually every cell in the body. It continuously enters the bloodstream and is subsequently filtered and broken down by the kidneys. Because this process is constant and stable, the blood concentration of cystatin C is a good indicator of how well the kidneys are performing their filtering task.

What makes cystatin C special compared to other kidney function markers, such as creatinine, is that the value is less strongly influenced by muscle mass or diet. This makes the measurement reliable in people with low muscle mass, such as the elderly or people who have been ill for a long time.

Based on the cystatin C value, the estimated glomerular filtration rate (eGFR) can also be calculated. This number indicates how much blood the kidneys filter per minute and is a direct measure of kidney function.

What is Cystatin C measured for?

The measurement of cystatin C is used to assess kidney function. The kidneys play a crucial role in the body: they filter waste products from the blood, regulate fluid balance, and keep blood pressure stable. If the kidneys function less effectively, substances that the body normally excretes gradually accumulate.

A cystatin C test is particularly useful when the result of a creatinine measurement is unclear or unreliable. This may be the case for people with an atypical physique, for the elderly, or when a doctor wants a second confirmation of a previously measured kidney function value.

In addition, cystatin C is sometimes used in people with an increased risk of kidney damage, for example due to long-term use of certain medications, diabetes mellitus, or long-standing high blood pressure.

What do abnormal values ​​mean?

Elevated cystatin C value

When cystatin C is higher than the reference value, this usually means that the kidneys are filtering protein less efficiently. This can be a sign of impaired kidney function. The higher the value, the greater the chance that the kidneys are under pressure or have been damaged.

However, an elevated value is not always the result of a kidney disorder. Cortisone use can increase the production of cystatin C, causing the measurement to show a higher value than would be expected based on actual kidney function. The same applies to pregnancy and to an overactive thyroid, also known as hyperthyroidism. When interpreting the result, it is therefore important that the doctor takes these factors into account.

Low cystatin C value

A low cystatin C value is rare and in most cases has no clinical significance. However, in some situations, a reduced value may occur with an untreated underactive thyroid, also known as hypothyroidism. In that case, the body produces less cystatin C, which can lead to an overestimation of kidney function via the calculated eGFR. The kidneys then appear to be working better than they actually do.

Symptoms of impaired kidney function

Kidney problems often develop insidiously. In the early stages, there are rarely any symptoms because the kidneys have a large reserve capacity. Only when kidney function has significantly declined can waste products accumulate in the blood and symptoms appear.

Possible symptoms of chronically impaired kidney function include:

• Reduced appetite
• Nausea or vomiting
• Fatigue that does not subside with rest
• Itchy skin
• Weakened muscle strength

Because these symptoms can also occur in many other conditions, blood tests are essential to objectively assess kidney function. Early detection offers the best opportunities to limit further deterioration.

When should you have Cystatin C levels measured?

You can have Cystatin C levels measured if you want to know how your kidneys are functioning, even without experiencing any symptoms. A preventive measurement is useful in the presence of risk factors such as diabetes, high blood pressure, being overweight, or a family history of kidney disease.

Even when a doctor has previously detected an abnormal creatinine value, a cystatin-C measurement can provide additional clarity. The two markers complement each other and together provide a more reliable picture of kidney function than either individually.

Always discuss the results with a doctor. Values ​​outside the reference range require careful interpretation in light of your overall health, medication use, and any other conditions.