CRP blood test guide: what an inflammation marker can and can’t tell you

CRP — short for C-reactive protein — is a protein made by the liver that rises when there’s inflammation in the body. It’s one of the most commonly used inflammation markers in UK primary care, and it’s often included in private health checks.

Here’s the most important thing to know about CRP, and it’s worth saying up front: a CRP test can tell you that inflammation is present, but it can’t tell you what’s causing it. Almost anything that causes inflammation — from a minor infection to an autoimmune condition to recent intense exercise — can raise CRP. The number on the report is a signal, not an answer.

If your CRP has come back raised, please read this guide as background information and also speak to your GP. The cause of a raised CRP matters far more than the number itself, and only a clinician with your full clinical picture can put it in context.

What CRP measures

CRP is an acute-phase protein. It’s produced by the liver in response to inflammatory signals from elsewhere in the body, and it can rise within a few hours of inflammation starting. When the inflammation settles, CRP usually falls again within days.

That makes it useful as a quick, broad signal — but also non-specific. CRP doesn’t tell you where the inflammation is, what kind it is, or what’s causing it. A raised CRP after a winter cold and a raised CRP from an autoimmune flare can look very similar on a blood report.

You may also see ESR (erythrocyte sedimentation rate) mentioned alongside or instead of CRP. ESR is an older inflammation marker that measures something different but reflects similar processes. The two are sometimes interpreted together. CRP is generally preferred in modern UK practice for most purposes because it rises and falls more quickly.

Standard CRP vs high-sensitivity CRP (hs-CRP)

This is one of the most confusing things about CRP testing, and it’s worth taking a moment on.

Standard CRP is the test used in acute clinical settings — for example by a GP or hospital looking for significant inflammation from infection, an autoimmune flare, or a serious illness. The typical reference range varies between labs, but standard CRP is usually reported as “under about 10 mg/L” in healthy adults. Results higher than that are interpreted in the context of symptoms and other findings.

High-sensitivity CRP (hs-CRP) is the same protein, but measured with a more sensitive test that can detect much lower levels. It’s used to look at low-grade, longer-term inflammation rather than acute illness. In research, hs-CRP has been studied as a marker of chronic inflammation linked to cardiovascular risk, and you may see “lower”, “average” and “higher” population risk categories described in research literature.

These categories are research thresholds applied at population level. They aren’t personal targets, and they don’t predict any individual’s risk of a heart attack or stroke. We come back to this in the cardiovascular section below.

Private health checks that include CRP often use hs-CRP precisely because most asymptomatic adults will have CRP levels well below the standard test’s reporting range, and hs-CRP gives a more useful signal in that context.

What raised CRP may suggest

A raised CRP simply means inflammation is present somewhere in the body. The list of possible causes is wide, which is exactly why the result needs context:

• Acute infection — bacterial infections typically raise CRP more than viral, but both can do it.
• Autoimmune and inflammatory conditions — for example rheumatoid arthritis, inflammatory bowel disease (IBD), and lupus. CRP is often used to monitor these conditions under specialist care.
• Chronic low-grade inflammation — sometimes linked to obesity, smoking, a sedentary lifestyle, or features of metabolic syndrome.
• Recent injury, surgery or trauma — including minor procedures.
• Dental infection — a surprisingly common and often overlooked cause of low-level inflammation.
• Recent vigorous exercise — a hard workout in the day or two before a blood test can raise CRP modestly.
• Pregnancy — CRP rises naturally in pregnancy.
• Cardiovascular events — heart attacks and other significant cardiovascular events involve inflammation.
• Some cancers — raised CRP can occur alongside some cancers, but CRP is not a cancer screening test and, in practice, a raised result is far more likely to reflect one of the other causes above.

That list isn’t a checklist to work through on your own. CRP doesn’t differentiate between these causes — it just shows inflammation is happening. Working out what kind of inflammation, and what to do about it, is a clinical assessment that involves your symptoms, examination, history and often other targeted tests.

If your CRP is significantly raised and you don’t have an obvious recent infection, please see your GP. They can examine you, ask about symptoms, and decide what (if anything) needs to happen next.

What a normal or low CRP means

A normal or low CRP suggests that significant inflammation is unlikely at the time of the test. That’s reassuring, but worth understanding for what it is and isn’t.

A normal CRP doesn’t rule out all health problems. Many conditions can exist without raising CRP. Some autoimmune conditions — lupus is a notable example — can flare significantly without much change in CRP. That’s why CRP is interpreted alongside other markers and clinical findings, not on its own.

If you have ongoing symptoms that are worrying you, a normal CRP doesn’t mean the symptoms aren’t real or don’t need investigating. The number on the test isn’t a verdict on your health.

CRP and cardiovascular risk

hs-CRP has been studied extensively as a marker of chronic low-grade inflammation, and at the population level, raised hs-CRP is associated with a higher risk of heart disease and stroke. It’s one of the reasons CRP appears in many private health check panels.

A few important caveats are worth being clear about:

• hs-CRP isn’t used in isolation in UK clinical guidance. The main cardiovascular risk calculators used in UK practice focus on factors such as age, blood pressure, cholesterol, smoking and diabetes rather than CRP.
• The hs-CRP risk categories are population averages, not personal predictions. Someone with hs-CRP in the “higher” category isn’t destined for a cardiovascular event, and someone in the “lower” category isn’t immune from one.
• The established cardiovascular risk drivers matter more than CRP in isolation. Blood pressure, cholesterol, blood sugar, weight, smoking and family history are still the things that drive individual risk and that responding to is most likely to help.

hs-CRP is best thought of as one additional piece of information that fits into a wider picture, not as a standalone cardiovascular score.

What CRP results can’t tell you

CRP results:

• Can’t tell you the cause of inflammation. A raised CRP shows inflammation is present, but not where, what kind, or why.
• Can’t reliably distinguish acute from chronic inflammation on a single test. The trend over time, alongside symptoms, is more informative than one number.
• Can vary day to day. A heavy workout, a minor cold, a dental issue or a recent injury can all temporarily raise CRP without it meaning a long-term problem.
• Don’t rule out all health problems if normal. Some serious conditions don’t raise CRP much, and some autoimmune flares (notably lupus) can be active without significantly raised CRP.
• Aren’t a personal cardiovascular risk score. Population-level associations between hs-CRP and cardiovascular risk don’t translate directly into individual predictions.
• Don’t replace your GP’s judgement. Only a clinician with your full history can put a raised CRP in context.

If your CRP is outside the reference range, the next step is a GP appointment — not extensive online research about specific conditions. Your GP can examine you, ask about symptoms, and arrange targeted follow-up tests if needed.

Fasting, preparation and timing

A few practical points worth knowing:

• Fasting isn’t required for a CRP test. You can eat and drink normally before the test. For broader prep advice, see our eating before a blood test guide.
• Recent vigorous exercise can raise CRP modestly. If you want a baseline measurement, avoiding a heavy workout the day before the test gives a slightly cleaner reading.
• Recent illness affects results. A cold, sore throat, stomach bug or any other infection in the days before the test will likely affect your CRP. If you’ve been unwell, your result may not reflect your usual baseline — let the clinic know if relevant.

Should you book a single CRP test or a broader health check?

This depends on what you’re trying to understand.

A single CRP test may make sense if:

• You’re tracking inflammation under specialist guidance — for example, monitoring an autoimmune condition where your specialist team has asked for periodic CRP measurements.
• You want a focused recheck on a previously raised result.
• You already have a wider clinical picture in place.

You can read more on the CRP blood test page.

A broader health check is often more useful for asymptomatic adults because CRP is most meaningful alongside other markers. The Optimal Health Check includes hs-CRP alongside a wider blood panel — full blood count, kidney function, liver function, cholesterol, HbA1c, thyroid function — and in-clinic checks that help build a picture of overall metabolic and cardiovascular health.

The Advanced Health Check is also worth considering as a step up from the Essential. Our 5 essential blood tests for adults guide and why health screening matters post give wider context if you’re new to private blood testing.

If you’re not sure which option suits you, contact us and we can help you choose.

When to speak to your GP

Speak to your GP if your CRP is outside the reference range, especially if you don’t have an obvious recent infection that would explain it. A persistent raised CRP without a clear cause needs clinical assessment.

Speak to your GP about symptoms, not just numbers. Things worth raising include:

• Ongoing joint pain or stiffness
• Persistent unexplained fatigue
• Recurrent or persistent fever
• Unexplained weight loss
• Persistent digestive symptoms — changes in bowel habit, ongoing abdominal pain, blood in stools
• Persistent skin rashes, particularly with other systemic symptoms
• Any symptom that’s worrying you, regardless of what your CRP shows

Symptoms matter more than the number on the report. A normal CRP doesn’t mean your symptoms aren’t real, and a raised CRP without symptoms still needs the cause investigated.

If you have an autoimmune or chronic inflammatory condition and your CRP is rising, contact the specialist team managing your condition rather than waiting for a routine appointment.

A private health check is designed for adults who currently feel well or want a baseline. It can usefully complement NHS care and help you prepare for a more focused conversation with your GP. It doesn’t replace medical care if you’re unwell.

In a life-threatening emergency — severe pain, high fever with confusion, severe shortness of breath, or signs of sepsis — call 999 immediately. For urgent advice that isn’t an emergency, NHS 111 is available online or by phone.

A signal, not an answer

CRP is genuinely useful because it gives a fast, broad signal that inflammation is present. That can prompt useful conversations, useful follow-up tests, and useful changes in management. It can also reassure when it’s normal in someone without significant symptoms.

But CRP is one of the least specific blood tests in common use. The number on its own doesn’t tell you what’s causing the inflammation, doesn’t tell you whether it’s serious, and doesn’t tell you what to do about it. The most useful interpretation comes from looking at CRP alongside symptoms, examination, history and other tests — which is GP territory.

If you’d like to check CRP as part of a broader review, the Optimal Health Check is the natural fit. If you’re unsure which option is right for you, contact us and we can help you find the most appropriate starting point. Appointments are available from our Kingston upon Thames and Crawley clinics.