C-reactive protein (CRP) test: what it measures, normal levels, high-sensitivity CRP, and what a high CRP means

C-reactive protein is an acute-phase protein: when immune cells detect infection, injury or inflammation they release interleukin-6, and the liver responds by producing CRP. Blood levels begin to rise within about six hours, can increase a thousandfold within two days, and fall back quickly once the trigger resolves because the protein's half-life is only about 19 hours. That fast rise-and-fall is what makes CRP useful — it tracks inflammation almost in real time.

The test measures CRP concentration in milligrams per liter (mg/L); some US labs still report milligrams per deciliter (mg/dL), where 1 mg/dL equals 10 mg/L. It does not tell you where the inflammation is or what is causing it. A pneumonia, a flare of rheumatoid arthritis, a fresh surgical wound and a heart attack can all produce similar numbers.

There are two versions of the assay. Standard (conventional) CRP is designed for the high range seen in infection and inflammatory disease. High-sensitivity CRP (hs-CRP) measures the same protein but precisely at the low end, below 10 mg/L, where small chronic differences relate to cardiovascular risk. They are the same molecule, reported for different purposes.

For the standard test, most laboratories consider values under 10 mg/L (1.0 mg/dL) to be within the reference range; many healthy adults run under 3 mg/L. Above 10 mg/L is generally taken to indicate clinically significant inflammation. Values between 10 and 40 mg/L are typical of viral infections and mild inflammatory conditions, 40 to 100 mg/L of bacterial infections and active inflammatory disease, and values above 100 mg/L point toward a severe bacterial infection, major trauma, or a widespread inflammatory process.

For hs-CRP used in cardiovascular risk assessment, the American Heart Association and the CDC define three bands: under 1 mg/L is lower risk, 1 to 3 mg/L is average risk, and above 3 mg/L is higher risk. Those bands only apply when the person is otherwise well; a cold, a sprained ankle or a dental infection will push hs-CRP above 3 without saying anything about the heart, which is why the guideline recommends two measurements about two weeks apart with the lower value used.

If the question is whether you currently have a significant infection or an active inflammatory condition, the standard CRP is the right test — it is cheaper and covers the range that matters. If the question is your long-term cardiovascular risk and you feel well, hs-CRP is the right test, and it is only meaningful when you have not been ill or injured in the previous two weeks.

Many labs now run a single assay that covers both ranges and report the result once. In that case the same number is read against both scales: a value of 2 mg/L is unremarkable for infection and sits in the average cardiovascular band. TestWell offers both a standard CRP and an hs-CRP test; the hs-CRP page covers the cardiovascular interpretation in more depth.

Almost anything that activates the immune system raises CRP. The causes below are ordered roughly by how often they explain a raised result in people who get tested:

Erythrocyte sedimentation rate (ESR, or sed rate) is the older inflammation test. It measures how fast red cells settle in a tube, which depends on fibrinogen and immunoglobulins, so it rises more slowly — over days — and stays up for weeks after the inflammation has resolved. It is also skewed by anemia, pregnancy, age and kidney disease independently of inflammation.

CRP responds within hours and normalizes within days, so it is better for detecting acute inflammation and for judging whether treatment is working quickly. ESR remains useful for a few conditions where it tracks disease better than CRP — temporal arteritis, polymyalgia rheumatica, lupus and some bone infections — which is why the two are often ordered together. A high CRP with a normal ESR usually means something recent; a high ESR with a normal CRP suggests something longer-standing or one of the ESR-favoring conditions.

A raised CRP is a signal to find the cause, and the cause is usually obvious from the story: a recent illness, an injury, a flare of a known condition. When it is not, a clinician will look for infection (symptoms, examination, sometimes a CBC and urinalysis) and for inflammatory disease (joint or gut symptoms, rashes, ESR and autoimmune markers). If nothing is found and the value is modest, repeating it in two to four weeks is often the right move — transient elevations from minor infections are common and resolve on their own.

A persistently elevated hs-CRP in someone who is well is best treated as a risk marker rather than a disease. The evidence-based responses are the ones that lower cardiovascular risk generally — not smoking, regular activity, weight loss where relevant, and management of blood pressure, cholesterol and blood sugar — and a discussion with a clinician about whether a statin is appropriate, since statins lower both LDL and CRP. Guidelines do not recommend treating the CRP number itself.

CRP can be measured at any time of day and does not require fasting. Because intense exercise raises it for a day or two and any acute illness raises it for a week or more, the most interpretable draw is one taken when you are well and rested.