Cystatin C: a more accurate look at your kidney function

Why measure kidney function?

Your kidneys filter waste products out of your blood day and night. How well they do this is expressed in the filtration rate (GFR). Reduced filtration often causes no symptoms at first, so it can go unnoticed for a long time. A blood test is therefore the usual way to assess kidney function, especially with high blood pressure, diabetes or long-term medication use.

The problem with creatinine alone

The most widely used marker is creatinine, a waste product formed by the breakdown of creatine in the muscles. The eGFR, an estimate of the filtration rate, is calculated from creatinine, age and sex. This works well for most people, but has one important limitation: the creatinine value strongly depends on how much muscle mass you have.

Someone with high muscle mass, such as a strength athlete, naturally produces more creatinine. The value can then be elevated without anything being wrong with the kidneys. Conversely, someone with low muscle mass, for example at an older age or a low body weight, may have a misleadingly reassuring value. In both cases the creatinine-based eGFR is less reliable.

What is Cystatin C?

Cystatin C is a small protein produced at a constant rate by virtually all cells in the body. It is filtered out of the blood by the kidneys. If the kidneys filter less well, the amount of cystatin C in the blood rises.

What makes it special is that its production barely depends on muscle mass, sex or diet. As a result, cystatin C gives a more stable and often more accurate picture of the filtration function than creatinine alone.

When is Cystatin C useful?

Cystatin C is especially useful when the creatinine value is difficult to interpret. Think of:

• people with high muscle mass, such as strength athletes and bodybuilders
• people with low muscle mass, such as older people and people with a low weight
• a creatinine or eGFR result that is just abnormal or doubtful

In those situations cystatin C helps to confirm whether there really is reduced filtration function.

For athletes there is an extra advantage: whereas a heavy training session or the use of creatine supplements can temporarily raise the creatinine value, cystatin C is barely affected by this. That gives you a clean picture of kidney function, regardless of your training schedule.

How do you read the result?

Cystatin C is measured in milligrams per litre (mg/L). The laboratory states the reference values that apply there alongside your result. An elevated cystatin C may point to reduced filtration function.

Even so, a single measurement is not a diagnosis: a strongly overactive thyroid, inflammation or the use of corticosteroids can also influence the value. For kidney function, the trend over time is often more important than a single value.

Testing at Bloodworks

At Bloodworks you have cystatin C drawn at one of the 1000+ collection points in the Netherlands. Every result is reviewed by a doctor and delivered as a personal PDF report, with an explanation of the value and, where applicable, the advice to contact your GP.

Cystatin C can be ordered separately and added to the same blood draw as a package that already includes creatinine and eGFR, such as Bloodworks Sport. That gives you a fuller picture of kidney function.