Best Tests September 2005
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When requesting CRP or ESR, bpacnz recommends:
- Choose CRP first on most occasions
- Seldom request ESR and CRP simultaneously
CRP is a useful test when considering an atypical infection, as it can help differentiate between bacterial and viral infections. As the CRP increases above 100 mg/L, the likelihood of a bacterial infection becomes greater than a viral infection.
Screening asymptomatic patients
CRP and ESR are not suitable for screening asymptomatic patients. When there is no strong evidence of disease, the tests are of little value.
Both CRP and ESR are recommended in the diagnosis of PMR while CRP is recommended for monitoring. The initial CRP level should be used as the baseline for monitoring treatment, therefore an absence of a clinical response and an improvement of the CRP within one to two weeks of therapy, should suggest reconsideration of the diagnosis.
Temporal (giant cell) arteritis
While overall there is good correlation between CRP and ESR at the diagnosis of temporal arteritis, cases with normal ESR and elevated CRP are reported. It is therefore recommended that CRP and ESR should be tested simultaneously, which will result in a higher sensitivity for diagnosis. CRP is also recommended for the monitoring of temporal arteritis, and appropriate treatment would show an improvement in the CRP level.
Neither CRP or ESR are included in the diagnostic criteria for rheumatoid arthritis. However CRP should be used for monitoring because it is a better measure of the disease activity and it is known that sustained high levels of CRP are associated with worse outcomes.
Systemic lupus erythematosus (SLE)
There is a lack of correlation between CRP and disease activity in SLE. A more useful role of CRP is to distinguish between a lupus flare and infection: it usually remains normal in a flare but is elevated in infection, while the ESR is often elevated in both.
Given the non-specific nature of the acute phase response, a definite role of CRP measurements in the management of cancer patients has not yet been established, other than in cases of intercurrent infection. Neither CRP nor ESR should be used as a screening test for malignancy in the general population, since any increase in these is non-specific.
CRP as a cardiovascular disease risk factor
High Sensitivity-CRP (Hs-CRP) has been reported as predictor for cardiovascular disease, although when compared with major established risk factors (such as elevated cholesterol and smoking status) the Hs-CRP added only marginally to the predictive value of the current risk factors.