High CRP: how to read your C-reactive protein result

CRP (C-reactive protein) is made by the liver within hours of inflammation or infection. In most UK labs a normal CRP is under 5 mg/L (some use under 10). Many US labs report in mg/dL, where the same normal limit reads as under 0.5 or 1.0: a CRP of 1.0 mg/dL equals 10 mg/L. Always check the unit before deciding whether your number is high.

Reference ranges

Category	Reference range
Normal	under 5 (under 0.5) mg/L (mg/dL in brackets)
Mild rise: minor infection, inflammation	5–40 (0.5–4) mg/L (mg/dL in brackets)
Bacterial infection more likely	often above 50–100 (5–10) mg/L (mg/dL in brackets)
hs-CRP, heart risk test	under 1 low, 1–3 average, over 3 higher mg/L (mg/dL in brackets)

Ranges and cut-offs vary by lab and situation; use the range on your report. To convert, mg/L ÷ 10 = mg/dL.

What a high CRP means

CRP is an acute-phase protein: levels can multiply within 6 to 8 hours of an infection or injury and fall quickly once the cause settles, roughly halving every day. That makes it useful for spotting inflammation and for following whether an infection is responding to treatment.

Like ESR, CRP is not specific. It tells your doctor that inflammation is present and roughly how much, not where it is or what is causing it.

CRP mg/L vs mg/dL: the most common confusion

Because of the units, "CRP 10" can mean very different things. A UK report showing 10 mg/L describes a mild rise. A US report showing 10 mg/dL means 100 mg/L, a substantial rise that usually reflects a significant infection or inflammation.

There is also a separate test, high-sensitivity CRP (hs-CRP), used in some countries to help estimate heart disease risk in people who are otherwise well. It measures very low levels (under 1 to about 10 mg/L) and should not be confused with a standard CRP taken when you are ill.

High CRP causes

• Infections: bacterial infections usually raise CRP more than viral ones, although there is overlap.
• Inflammatory conditions: rheumatoid arthritis, inflammatory bowel disease, vasculitis, pancreatitis.
• Tissue injury: surgery, fractures, burns, a heart attack.
• Long-term low-grade causes: obesity, smoking, poorly controlled diabetes, poor sleep, which tend to keep CRP mildly raised (often 3–10 mg/L).
• Pregnancy, which can raise CRP slightly.
• Some cancers, less commonly.

High CRP with normal ESR

CRP rises within hours; ESR takes days. So a high CRP with a normal ESR often means an inflammation that has just started, such as an early infection. The opposite pattern, high ESR with normal CRP, is more typical of recovery or of conditions that affect red cells and blood proteins. Your doctor will read the two together with your symptoms and, often, with the white cell count.

Following CRP over time

Because CRP changes so quickly, a single value is often less informative than the direction of travel. In hospital, CRP is commonly rechecked every one to three days during an infection: a fall of about half or more suggests treatment is working, while a rising value prompts a rethink. After surgery, CRP normally peaks around day two or three and then falls; a second rise later on can be an early sign of a complication.

Outside hospital, a mildly raised CRP after a cold or a flare of a known condition is often rechecked once you feel better. If it has settled, no further action is usually needed. If it stays raised without a clear reason, your doctor may look for a hidden source of inflammation.

When to see a doctor

Any raised CRP should be discussed with the doctor who requested it, and mild rises are often simply rechecked. Get urgent help (NHS 111, urgent care, A&E or the emergency room) if a high CRP comes with:

• fever, shivering, a fast heartbeat, fast breathing, confusion or mottled skin: possible sepsis;
• severe abdominal pain, chest pain or difficulty breathing;
• a hot, red, swollen joint, especially with fever;
• a very high value, for example above 100 mg/L (10 mg/dL), and you feel unwell.

Sources

Written using public information from:

• Lab Tests Online UK (Association for Clinical Biochemistry and Laboratory Medicine): CRP
• MedlinePlus (US National Library of Medicine): C-reactive protein test
• NICE (National Institute for Health and Care Excellence): Pneumonia in adults; Sepsis
• American Heart Association: inflammation and heart disease

This material is for general information only. It is not a medical opinion and does not replace a consultation with a doctor. Reference ranges depend on the laboratory and the method — rely on the range printed on your own report. Your doctor makes the final interpretation.