CRP

CRP, short for C-reactive protein, is a protein that the liver produces whenever inflammation or infection occurs somewhere in the body. In a healthy person, the value is low, usually below 5 mg/L. A significantly elevated value means that the immune system is reacting to something. The CRP value itself does not reveal exactly what that is. That result always requires context: how does the person feel, and what do any other blood values ​​show?

Reference values

• Normal: below 5 mg/L — no indication of active inflammation
• Slightly elevated: 5–10 mg/L — mild inflammation possible, can also occur with a common cold
• Moderately elevated: 10–50 mg/L — consistent with a viral infection such as the flu, or an incipient bacterial infection
• Significantly elevated: 50–300 mg/L — more often indicates a bacterial infection or severe inflammation
• Very strongly elevated: above 300 mg/L — may indicate a severe infection or strong inflammatory response; requires medical assessment

What is CRP?

CRP is a protein that exists exclusively to support the immune system. As soon as the body detects a threat, it signals the liver to rapidly produce CRP and release it into the blood. The protein binds to damaged cells and pathogens, allowing other components of the immune system to clear them more quickly.

This makes CRP a reliable indicator: it rises rapidly when there is a real problem and drops just as quickly when the inflammation is under control. This makes it useful for both detecting a problem and monitoring treatment.

How does it work in the body?

When tissue is damaged or a pathogen enters, certain cells release signaling molecules, the best known of which is interleukin-6. These substances reach the liver via the blood. The liver responds by producing CRP within a few hours. The value can rise rapidly in that initial phase, sometimes from virtually zero to tens or hundreds of mg/L within one to two days. As soon as the cause disappears, the CRP value in turn drops relatively quickly.

Why is CRP tested?

CRP is tested to assess whether an inflammation or infection is active in the body. This is useful in various situations.

A general practitioner can use the test to determine whether symptoms such as fever, pain, or general malaise have a physical cause that requires attention. With a mild cold, the CRP value usually remains normal or is only slightly elevated. With a more serious infection, such as pneumonia or a complicated urinary tract infection, the value rises more noticeably. This distinction helps determine whether treatment is necessary.

In addition, CRP is used for known conditions such as rheumatoid arthritis or inflammation of the intestines. In those cases, a repeated measurement provides insight into whether the treatment is effective and whether the inflammation is decreasing or increasing.

A single measurement in itself rarely provides a complete answer. The value is only meaningful in combination with symptoms, medical history, and any other blood values.

Symptoms associated with an abnormal value

An elevated CRP value is often accompanied by symptoms consistent with inflammation or infection. Common signs include:

• Fever or elevated body temperature
• Fatigue and general malaise
• Pain or swelling in a specific area of ​​the body
• Redness and warmth around an inflamed area
• Sore throat, cough, or other respiratory symptoms associated with an infection
• Joint complaints associated with autoimmune diseases such as rheumatism

A low CRP value causes no symptoms and requires no action. It simply means that there is no indication of active inflammation at the time of sampling.

What influences the value?

Various factors can influence the CRP value, independent of an infection or inflammation.

Fasting is not strictly required for CRP measurement, but a recent meal can cause a slight increase in some people. The time of day generally has little influence on the result.

Age plays a limited role: in older people, the baseline value may be slightly higher, even without an identifiable cause. Being overweight and smoking are associated with a chronically slightly elevated CRP value.

Some medications have an influence. Anti-inflammatory drugs such as ibuprofen or prednisolone can lower the CRP value, even if the underlying inflammation is still present. Statins, which are prescribed for high cholesterol, can also affect the CRP value.

Finally, CRP also rises in cases of tissue damage without infection, such as after surgery, a heart attack, or a severe injury. In those situations, the high value reflects the body's recovery response, not necessarily an infection.