Relationship between CRP and clinical course of unstable angina depends on assay method

The CRP concentration has been identified as a cardiovascular risk factor in healthy volunteers and in patients with stable and unstable angina pectoris. There is little information about the comparability of the results of different CRP assays and it has also not been evaluated whether different assays give different associations with risk.

We studied this in the APRAIS study, a study on 211 patients with unstable angina pectoris who were admitted to hospital and in whom we studied the association between the CRP concentration on admission and the in-hospital clinical course (refractiviness to medication). We used two different immunological methods: an in-house ELISA using polyclonal rabbit anti-human CRP antibodies as catching and tagging antibody and a commercial nephelometric method (Dade-Behring).

The analytical variation was much smaller for the nephelometric method than for the in-house ELISA. Both methods gave higher levels in the refractory group than in the stabilized group, but only for the nephelometric method was this difference significant. Also, when the relative risk is calculated, it is clear that the more accurate nephelometric method can better discriminate between the two groups [RR 2.19 (95% CI 0.94-5.11) for the nephelometric method and RR 1.30 (95% CI 0.56-3.03) for the ELISA].

In conclusion, the nephelometric method measures the CRP concentration with a smaller intra- and interassay variation. The nephelometric method can also better discriminate between unstable angina patients who will be stabilized and those who are refractory during hospitalization.