Frequency and causes of C-reactive protein and erythrocyte sedimentation rate disagreements in adults

AimC-reactive protein (CRP) and the erythrocyte sedimentation rate (ESR) are often ordered together in patients with suspected infection or inflammation. However, the test results can disagree in as many as 33% of patients. Our aim was to further examine CRP/ESR disagreements and their stability on repeat testing. MethodsWe analyzed simultaneously ordered CRP and ESR results in 70 adult patients who had been tested on three separate occasions a median of 4weeks apart. ResultsInitial CRP/ESR disagreements occurred in 14 of the 70 patients (20%; 95% CI, 12-31). Only four of these 14 patients had stable CRP/ESR disagreements throughout the study (two with lupus nephritis and one with osteomyelitis had persistent high ESR/normal CRP disagreements and one with rheumatoid arthritis had a persistent high CRP/normal ESR disagreement). The other 10 patients with initial CRP/ESR disagreements later exhibited CRP/ESR agreements. Of the 56 patients with initial CRP/ESR agreements, only 10 developed a CRP/ESR disagreement (or disagreements) on subsequent testing. ConclusionCRP/ESR disagreements are common in clinical practice. Three mechanisms were identified to explain CRP/ESR disagreements: (i) slight fluctuations in the CRP and ESR around the upper limits of normal for these tests; (ii) development of an intercurrent illness; and (iii) different time courses of CRP and ESR elevations, in which the CRP rose and fell faster than the ESR.