An elevated plasma level of homocysteine (Hcy) and infection by Chlamydia pneumoniae (C. pneumoniae) have been suggested as independent risk factors for carotid atherosclerosis (CA) and coronary artery disease (CAD), but the mechanisms involved are unclear. We investigated the correlation between positivity for antibody to C. pneumonia (anti-C. pneumoniae) and the Hcy level in patients with CA and CAD. The total plasma homocysteine (tHcy) concentration was determined in 99 patients with CA and 31 patients with CAD, as well as 119 controls with matched risk factors for atherosclerosis. The tHcy level was measured with a Bio-Rad microplate enzyme immunoassay. In the CAD group, the tHcy level (13.67 μmol/l) was significantly higher than that in other groups (CA group, 10.96 μmol/l; control group, 9.95 μmol/l; ANOVA, P = 0.0006). Positivity for anti-C. pneumoniae IgG was significantly more common in the CAD group (77.4%) than in the other groups (CA group, 53.5%; control group, 54.6%; ANOVA, P = 0.0490). There was no association between anti-C. pneumoniae IgA positivity or tHcy and conventional risk factors. However, anti-C. pneumoniae IgG positivity was significantly more common in subjects with higher tHcy levels than in those with low tHcy levels from each of the 3 groups. The CAD group had significantly higher tHcy levels, and tHcy was significantly associated with anti-C. pneumoniae IgG positivity. These findings indicate that elevation of tHcy is related to positivity for anti-C. pneumoniae IgG in patients with CAD.
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