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Lab test update - C-reactive protein (CRP)

In the second part of our new series, chemical pathologist Dr Stuart Smellie answers questions on a case involving C-reactive protein

In the second part of our new series, chemical pathologist Dr Stuart Smellie answers questions on a case involving C-reactive protein

The case

An anxious 75-year-old lady – a frequent attender – has a history of diverticular disease and IBS. She presents with alternating diarrhoea and constipation but isn't clear whether this represents a significant change in her symptoms. Further questioning and examination are unrewarding. To reassure her – and yourself – you arrange an FBC and ESR. The report a week later states that the lab no longer performs ESRs – it has tested the CRP instead, which is normal.

What is CRP?

CRP is C-reactive protein, a circulating protein that rises in the ‘acute phase reaction', the body's reaction to an acute inflammatory or infectious process. It is therefore quite a sensitive, but very non-specific, way of telling if someone is likely to have organic illness.

When is a CRP useful and what does an elevated value suggest?

CRP can be useful to discriminate between an inflammatory or infectious process and a functional or mechanical one, such as irritable bowel versus inflammatory bowel disease, or to distinguish acute infection from inflammation – it usually rises faster and higher in infection. If a lab test is required to monitor a condition, CRP can be a useful aid, although in most situations we monitor response to treatment using the patient's clinical state.

In our case study, for example, a high CRP might have suggested the patient had developed diverticulitis.

How reassuring is a normal value?

A normal value means an inflammatory or infectious process is unlikely to be present, although CRP may not be raised in low-grade inflammation or, for example, in early malignancy, so a normal value is of most use in providing reassurance if you think the patient may not have organic disease. But if this patient had not had IBS or diverticular disease, the history would suggest a need for urgent exclusion of malignancy irrespective of the CRP.

Is CRP of value as a cardiovascular risk marker?

The reference range for CRP is up to 5U/l. Within the normal range, however, higher CRP values (more than 2U/l) have been found in epidemiological studies to correlate with increased cardiovascular risk, possibly because they reflect some underlying inflammation in the arteries. But the jury is still out on this because once other factors, particularly weight, are corrected for it seems to have rather limited independent predictive value. We do not yet offer this test routinely as it would mean changing to a CRP method that could reliably measure these low values.

Are there any circumstances when an ESR remains a more valuable test than the CRP?

ESR is a good old-fashioned ‘bucket' lab test – one of the oldest – and measures how fast red cells settle in anticoagulated blood. It therefore reflects levels of the different blood proteins, which rise in the acute phase response, particularly fibrinogen and gamma globulins. These change red cell charges and make them sediment more quickly.

Unlike ESR, CRP is not typically raised by the presence of paraprotein bands as in myeloma. Also CRP is an early inflammatory marker and rises and falls faster than the other acute phase proteins, so it can rise before the ESR does.

For this reason it is often used to detect acute disease and as an early discriminator of bacterial infection from inflammation as it tends to rise more in infection.

On the other hand, ESR rises more slowly and is more often used as a lab marker to monitor chronic inflammatory conditions such as polymyalgia rheumatica. Like CRP, moderately raised values can be difficult to interpret. ESR also rises with age and in anaemia, causing further problems in interpreting results.

Because of this, some haematologists prefer measurement of serum viscosity instead, as this is independent of age, although the test method is less widely available.

Personally I think labs should offer both CRP and either ESR or viscosity.

Dr Stuart Smellie is consultant chemical pathologist for County Durham and Darlington Acute Hospitals NHS Trust, and associate director of the specialist interests section of the Association for Clinical Biochemistry

Competing interests None declared

CRP has the potential to be used as a CVD marker but current lab tests are not sensitive enough CRP has the potential to be used as a CVD marker but current lab tests are not sensitive enough

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