Association of C-Reactive Protein With Blood Pressure

Abstract

Davey Smith et al have used instrumental variable regression to assess the effect of C-reactive protein (CRP) on blood pressure (BP) in a cross-sectional sample of women.1 By using the 1059G/C polymorphism of the CRP gene as an instrument, they attempted to avoid confounding by unmeasured variables and reverse causality bias and concluded that elevated CRP levels do not lead to high BP. However, this analysis has several limitations. First, the 1059G/C polymorphism is a poor instrument because its effect on CRP level is small. Reported differences in CRP levels between White subjects with the GG and the GC variants were 0.33 mg/L in 1 study,2 0.07 mg/L in another (GG versus GC+CC),3 and nonsignificant in 3 others.4–6 In fact, in the author’s own study the average between-genotype CRP difference was only 0.42 mg/L.1 In the only prospective study on the association of CRP and incidence of hypertension,7 the smallest average difference in CRP level associated to a statistically significant increase in risk was 1.10 mg/L, almost 3× larger than that reported by Davey Smith et al.1 Second, the findings from the …