C-Reactive Protein (CRP): What This Acute Inflammation Marker Means

C-reactive protein is an acute-phase reactant made by hepatocytes in response to IL-6, the master inflammatory cytokine. CRP rises within 6 to 8 hours of an inflammatory trigger, roughly doubles every 8 hours, and peaks around 48 hours. Once the trigger resolves, the half-life is about 19 hours, so levels drop by half each day. That fast turnover is why CRP is the marker of choice for following infection in real time.

The protein itself does work. It binds phosphocholine on dead and dying cells and on certain bacteria, then tags them for complement and phagocytosis. So CRP is part of the response, not just a sign of it. Clinically, we read it as a thermometer.

Standard CRP reports values down to about 5 mg/L. Below that, the assay is imprecise. For any low-value question, especially chronic cardiovascular risk, you need hs-CRP.

The cutoffs below apply to standard (not high-sensitivity) CRP and reflect general clinical practice:

• Below 10 mg/L: normal or low-level inflammation. Most healthy adults run under 3 mg/L. Values between 3 and 10 are common with mild illness, recent exercise, obesity, or smoking.
• 10–40 mg/L: moderate inflammation. Viral infections, mild bacterial infections, post-surgery recovery, active autoimmune disease.
• 40–200 mg/L: significant inflammation. Active bacterial infection, major autoimmune flare, deep abscess, or substantial tissue injury.
• Above 200 mg/L: severe inflammation. Bacterial sepsis, severe pneumonia, large abscess, vasculitis flare, or major surgical recovery.

Context shifts everything. A CRP of 35 in a previously healthy person with a new fever is much more worrying than a CRP of 80 in someone three days post-knee-replacement.

Source: standard clinical chemistry reference; values vary by lab and assay.

Mildly elevated (10–40 mg/L)

A long list of common, mostly self-limited causes:

• Viral infections (influenza, COVID, EBV, common URIs).
• Mild bacterial infections (uncomplicated cystitis, mild cellulitis).
• Active but well-controlled autoimmune disease.
• Recent vaccination, especially in the first 24 to 48 hours.
• Post-surgical recovery in the first week or two.
• Tissue injury: sprain, fracture, deep bruise.

Moderately elevated (40–200 mg/L)

This is where CRP starts pointing at a process that needs treatment:

• Bacterial pneumonia, pyelonephritis, cellulitis with systemic signs.
• Cholecystitis, diverticulitis, appendicitis.
• Active rheumatoid arthritis, lupus flare, vasculitis, IBD flare.
• Deep tissue infection or abscess.
• Major surgery in the first 3 to 5 days.

Markedly elevated (above 200 mg/L)

Values this high almost always mean something serious. The differential narrows to bacterial sepsis, severe pneumonia, large or deep abscess, severe vasculitis, or significant trauma. A CRP over 200 in an unwell-appearing patient is one of the strongest single lab pointers toward bacterial infection.

Bacterial vs viral, the question CRP cannot quite answer

CRP does not cleanly separate bacterial from viral. There is too much overlap, especially in the 20 to 80 range. The extremes still help. A CRP under 20 in an adult with a respiratory illness is reassuring against bacterial pneumonia. A CRP over 100 makes bacterial infection likely enough that many clinicians treat empirically while cultures pend.

Tracking recovery

This is where CRP shines. Because it halves every day or so, two readings 24 hours apart tell you whether treatment is working. A CRP that drops from 180 to 90 on day 2 of antibiotics is a strong sign the right bug is being targeted. A CRP that stays at 180 or climbs means wrong drug, wrong diagnosis, or a complication (abscess, empyema) that needs drainage rather than more antibiotic.

Below 5 mg/L is normal and reassuring against any acute inflammatory process at the time of the draw. There is no clinically meaningful "too low" CRP on the standard assay.

One caveat. A low standard CRP says nothing about chronic low-grade cardiovascular inflammation. Someone reported as "less than 5" on the standard assay could still have an hs-CRP of 4.8 mg/L, which is a high cardiovascular risk reading. For chronic risk, order hs-CRP.

The one situation where a normal CRP can be misleading: severe liver failure. The liver makes CRP, so a sick septic patient with end-stage liver disease may not mount a CRP response. A "normal" CRP in obvious sepsis is not actually reassuring there.

Two readings 24 to 48 hours apart will tell you almost everything you need to know in an acute illness. The 19-hour half-life means meaningful change is visible within a day:

• Falling fast. The process is being controlled. After 48 hours of effective antibiotics for bacterial pneumonia, expect CRP to be roughly half of admission value.
• Falling slowly or flat. Possibly the wrong therapy, or a complication (abscess, septic joint, empyema) that needs drainage rather than more antibiotics.
• Rising. Process worsening or new complication. In a postoperative patient on day 5 to 7, a rising CRP after the expected post-op peak is one of the most reliable early signals of an infected collection.

Outside acute illness, a persistently mildly elevated CRP (10 to 30) over months suggests an ongoing inflammatory process worth investigating: smoldering autoimmune disease, untreated dental infection, occult abscess, inflammatory bowel disease, or (when paired with anemia) malignancy. For chronic cardiovascular risk specifically, switch to hs-CRP.

• CRP over 200, unwell-appearing. Assume bacterial infection or sepsis until proven otherwise. Same-day evaluation, blood cultures, broad-spectrum antibiotics where appropriate.
• CRP rising on day 5 to 7 post-op. Investigate for an infected collection or wound infection.
• CRP failing to fall on antibiotics. Reconsider the diagnosis, the drug, or whether source control (drainage) is needed.
• Persistently elevated CRP without an obvious cause. Systematic workup for autoimmune disease, occult infection, IBD, dental disease, and malignancy.
• CRP of 12 with no symptoms. Almost never a single-test crisis. Repeat in 2 to 4 weeks once any recent illness or workout has resolved.

• hs-CRP. Same protein, calibrated for low values. Use this for chronic cardiovascular risk, not acute illness.
• ESR. Slower up, longer memory. Useful in chronic disease (rheumatoid arthritis, polymyalgia rheumatica, giant cell arteritis) where a moving average is more interpretable than a snapshot.
• Fibrinogen. Another acute-phase reactant, and the protein that drives most of ESR's elevation. Tracks alongside CRP in inflammation.
• CBC with differential. Neutrophilia supports bacterial infection. Lymphopenia is common in viral illness. Pairing with CRP is more informative than either alone.
• Procalcitonin. More specific than CRP for bacterial infection, especially pneumonia. Slower to fall.
• Ferritin. Also an acute-phase reactant. Often co-elevated with CRP, and complicates iron-status interpretation during inflammation.