C-reactive protein (CRP) is an acute-phase protein made by the liver in response to inflammation, and the blood CRP level is the most widely used way to gauge how much inflammation is present in the body. Its production is driven largely by interleukin-6 released from inflamed, infected or injured tissue. CRP climbs within hours of a trigger and falls quickly once that trigger clears, so the number describes what is happening now, not last month. Two assays read the same molecule: the conventional test used to follow infection, and the high-sensitivity test that resolves the much smaller differences relevant to cardiovascular and aging research.
What it measures
CRP is measured in serum from an ordinary blood draw and reported in mg/L, though some US laboratories still report conventional CRP in mg/dL. The protein binds phosphocholine exposed on damaged cells and on some bacteria, activates complement and marks that debris for clearance, so it is part of the immune response, not only a readout of it. What it cannot do is tell you where the inflammation is or what caused it – a dental abscess, a flaring joint, a recent vaccination and visceral fat all raise the same number. The high-sensitivity assay is not a different substance, only a more precise reading at the low end of the scale. For a deeper reference, see the C-reactive protein (CRP) entry at Smart Longevity.
Typical values
| hs-CRP result | Conventional reading |
|---|---|
| Below 1 mg/L | Lower relative cardiovascular risk |
| 1 to 3 mg/L | Average relative risk |
| Above 3 mg/L | Higher relative risk |
| Above 10 mg/L | Points to acute inflammation or infection; the risk bands do not apply, so repeat the test once you are well |
Those bands come from long-standing cardiology and public-health guidance and are what most laboratories print alongside a hs-CRP — high-sensitivity C-reactive protein result. For conventional CRP the “normal” cut-off is not universal: it depends on the assay and the laboratory, so read it against your own report. Because one value bounces with any passing illness, standard practice is to average two measurements taken at least two weeks apart while you are well. Levels also run higher on average in women, with older age and with more body fat.
Why it matters for longevity
In large observational cohorts a higher hs-CRP is associated with greater risk of heart attack, stroke and all-cause mortality, and with frailty and cognitive decline in later life. Average levels drift upward with age, the measurable face of the low-grade inflammation called inflammaging, and CRP is one of the nine blood measures inside the PhenoAge biological-age formula. The honest caveat is that CRP looks like a messenger rather than a cause: genetic studies of variants that raise lifelong CRP have not shown a matching rise in heart disease. Blocking the pathway upstream does help – an antibody against interleukin-1 beta lowered hs-CRP and cut recurrent cardiovascular events after a heart attack without touching cholesterol – which is why CRP is read alongside the other Inflammatory markers rather than as a target in its own right.
What changes it
Losing visceral fat is the strongest everyday lever, because fat tissue secretes the interleukin-6 that tells the liver to make CRP. Aerobic and resistance training lower it, largely through fat loss, and so does stopping smoking. Treating a hidden source of inflammation – gum disease, sleep apnea, an active autoimmune condition – can move it more than any diet change. Eating patterns rich in fiber, vegetables, legumes and oily fish are associated with lower CRP, and statins lower it alongside LDL. Short sleep and sustained stress push it up. Plenty of things raise CRP without saying anything about your rate of aging: a cold, a recent vaccination, a hard endurance race, surgery, pregnancy and estrogen therapy. Hence the rule – do not test while you are unwell, and do not read one high number as a verdict.
Related reading
- Chronic Inflammation and Aging: Inflammaging
- Which Blood Tests Reflect Aging and How to Read Them
- What Biological Age Is and How to Measure It
Disclaimer. This article is for information only and does not replace medical advice. Talk to a qualified clinician before changing anything about your health.