CRP test: what the result means (CRB)

C-reactive protein is one of the body's acute-phase proteins: when inflammation starts anywhere — from an infection, an injury, an autoimmune condition or another process — the liver ramps up production within hours, and once the inflammation settles, CRP drops back down just as quickly. That makes CRP useful both for spotting inflammation and for tracking how it's responding to treatment. A separate version of the test, high-sensitivity CRP (hs-CRP), measures much smaller concentrations and is used to assess cardiovascular risk. The American Heart Association classifies hs-CRP up to 1 mg/l as lower risk, 1–3 mg/l as intermediate risk, and above 3 mg/l as higher cardiovascular risk. Normal CRP levels rise slightly with age, in women, and with obesity. CRP is often checked alongside ESR (erythrocyte sedimentation rate) and a full blood count.

• When infection or inflammation is suspected — fever, pain, swelling
• To monitor flare-ups of inflammatory conditions (rheumatoid arthritis, inflammatory bowel disease) and how well treatment is working
• To help distinguish a bacterial infection from a viral one — CRP is typically higher with bacterial infections
• To assess cardiovascular risk, using the high-sensitivity version of the test (hs-CRP)
• Alongside ferritin, to check whether a raised ferritin is being driven by inflammation

MedlinePlus classifies 0.3–1 mg/dl (3–10 mg/l) as a mild elevation whose clinical significance is unclear — it's neither a normal result nor a level that on its own points to serious inflammation or infection. Unlike the 10–100 mg/l or above-100 mg/l ranges, a result here more often reflects a mild or chronic influence than an acute process. It can be caused by many things unrelated to acute inflammation: obesity, smoking, hormone therapy, poor sleep, a mild cold, or even exercise shortly before the blood draw. Because of this, a single result in this range diagnoses nothing on its own — a doctor reads it together with your symptoms, and it is often repeated later or interpreted together with a full blood count and ESR, to see whether it's a one-off fluctuation or a persistent level pointing to chronic inflammation. If this result came from an hs-CRP test, a value above 3 mg/l falls into the AHA's higher cardiovascular risk category (MedlinePlus) — in that case it's read as a cardiovascular risk marker alongside other risk factors, not as a sign of inflammation on its own.

• Whether the result came from a standard or a high-sensitivity (hs-CRP) test — the two are interpreted differently
• Whether factors that can raise CRP without inflammation are present — obesity, smoking, recent exercise
• Whether it's worth repeating the test later if there are no symptoms
• How this result fits with the full blood count and ESR

MedlinePlus classifies 1–10 mg/dl (10–100 mg/l) as a moderate elevation, most often linked to infection or an inflammatory disease such as rheumatoid arthritis, inflammatory bowel disease or lupus. This range spans a tenfold difference, so a doctor usually looks at where exactly the result falls rather than just the fact that it's in range — 15 mg/l and 90 mg/l are both "moderate elevation," but the clinical picture can differ. A full blood count and ESR are usually checked alongside CRP to help distinguish a bacterial infection from a viral one or from chronic inflammation. CRP in this range tends to change quickly — rising within hours of inflammation starting and falling once it settles — so a single result is often rechecked after treatment begins to see whether it's coming down.

• The nature and duration of symptoms — fever, pain, swelling
• A full blood count and ESR alongside CRP
• Whether the result sits closer to the bottom or the top of the range
• When it's worth repeating the test to track how treatment is working

CRP above 100 mg/l: marked elevation

MedlinePlus classifies levels above 10 mg/dl (100 mg/l) as a marked elevation, most often reflecting a bacterial infection or severe inflammation. It's the highest of MedlinePlus's three categories, well above both the mild (3–10 mg/l) and moderate (10–100 mg/l) ranges, and is less often caused by mild or chronic processes — more often it points to an active, fast-developing inflammatory process. Because CRP responds quickly — rising within hours of onset — a result this high usually comes with obvious symptoms such as fever, significant pain, or a marked decline in how someone feels. A doctor assesses whether urgent care is needed and may order further tests, such as a blood culture, to find the source of infection. A number above 100 mg/l on its own doesn't give a diagnosis — it shows that an inflammatory process is significant, not where it's happening. The cause (infection, injury, autoimmune disease, or a rarer condition) is determined by a doctor based on symptoms, examination and other tests.

• Whether there's fever, severe pain, or other acute symptoms
• Whether urgent care is needed if the condition is worsening
• What further tests (e.g. a blood culture) could help identify the source
• When to recheck CRP to track how treatment is working

A raised CRP means there is inflammation somewhere in the body, but it does not say where it is or what is causing it. Most often the cause is an infection; other causes include rheumatoid arthritis, lupus, inflammatory bowel disease, rheumatism, a heart attack and cancer. In the MedlinePlus classification, 10–100 mg/l counts as a moderate elevation and above 100 mg/l as a marked one.

• Infection — bacterial, less often viral
• Rheumatoid arthritis
• Lupus and other autoimmune diseases
• Inflammatory bowel disease
• Rheumatism
• A heart attack
• Cancer
• Injury or surgery

Points usually weighed alongside a raised CRP: the symptoms and how long they have lasted — fever, pain, swelling; a full blood count and ESR, which together show the nature of the inflammation; when to repeat the test, since CRP falls quickly and so suits tracking how treatment is going; and, if the cause is unclear, which further tests are needed.

Low CRP: what it can mean

A low or undetectable CRP is normal — it means there was no active inflammation at the time of the test. A low CRP on its own does not indicate any illness: either there is no active inflammation, which is the normal state, or the inflammation has already passed and treatment is working. If symptoms persist while CRP is normal, discuss other possible causes with your doctor.