Chlamydia pneumoniae IgG titres and coronary heart disease: prospective study and meta-analysis.

Abstract

Objective To examine the association between coronary heart disease and serum markers of chronic Chlamydia pneumoniae infection.

Design "Nested" case-control analysis in a prospective cohort study and an updated meta-analysis of previous relevant studies.

Setting General practices in 18 towns in Britain.

Participants Of the 5661 men aged 40-59 who provided blood samples during 1978-80, 496 men who died from coronary heart disease or had non-fatal myocardial infarction and 989 men who had not developed coronary heart disease by 1996 were included.

Main outcome measures IgG serum antibodies to C pneumoniae in baseline samples; details of fatal and non-fatal coronary heart disease from medical records and death certificates.

Results 200 (40%) of the 496 men with coronary heart disease were in the top third of C pneumoniae titres compared with 329 (33%) of the 989 controls. The corresponding odds ratio for coronary heart disease was 1.66 (95% confidence interval 1.25 to 2.21), which fell to 1.22 (0.82 to 1.82) after adjustment for smoking and indicators of socioeconomic status. No strong associations were observed between C pneumoniae IgG titres and blood lipid concentrations, blood pressure, or plasma homocysteine concentration. In aggregate, the present study and 14 other prospective studies of C pneumoniae IgG titres included 3169 cases, yielding a combined odds ratio of 1.15 (0.97 to 1.36), with no significant heterogeneity among the separate studies ([chi square] = 10.5, df=14; P [is greater than] 0.1).

Conclusion This study, together with a meta-analysis of previous prospective studies, reliably excludes the existence of any strong association between C pneumoniae IgG titres and incident coronary heart disease. Further studies are required, however, to confirm or refute any modest association that may exist, particularly at younger ages.

Introduction

A study published in 1988 proposed that Chlamydia pneumoniae infection was an avoidable cause of coronary heart disease.[1] Since then, systematic reviews have identified several dozen additional studies of C pneumoniae markers and vascular disease.[2-4] Although some reports have suggested twofold or larger odds ratios for coronary heart disease in people with markers of chronic C pneumoniae infection, these studies have generally been small, retrospective, or liable to biases.[2-4] We report a study of 496 cases of coronary heart disease and 989 controls "nested" in a prospective cohort of British men monitored for 16 years. We also conducted an updated meta-analysis of other prospective studies to place our results in context.

Participants and methods

Cases and controls

During 1978-80, 7735 men aged 40-59 (response rate 78%) were randomly selected from general practice registers in each of 24 British towns and entered in the British Regional Heart Study.[5] Nurses administered epidemiological questionnaires, made physical measurements, and recorded an electrocardiogram. Nonfasting venous blood samples were collected in 5661 men in 18 of the towns and stored at - 20 [degrees] C for subsequent analysis. Further questionnaires were posted after five years (98% response among survivors) and 12 years of follow up (90% response among survivors) that asked about car ownership and childhood social circumstances (father's social class and childhood household amenities) respectively. All men have been monitored since entry for death from all causes and for cardiovascular morbidity, with a loss to follow up of less than 1%.[5] Cases in our study were men who had fatal coronary events or non-fatal myocardial infarction between the beginning of follow up and December 1995 and who had a stored serum sample available for analysis. Fatal cases of coronary heart disease were ascertained through NHS central registers on the basis of a death certificate with International Classification of Disease (ICD-9) codes 410-414. Non-fatal myocardial infarction was based on reports from general practitioners, supplemented by evidence from general practice records, meeting World Health Organization criteria.[5] Of 507 potential cases (223 deaths from coronary heart disease and 284 non-fatal myocardial infarctions), 496 had C pneumoniae measurements available. A total of 1026 controls, who were "frequency matched" to cases on town of residence and age in five year bands, were randomly selected from among men who had survived to the end of the study period without a myocardial infarction; 989 of these controls had C pneumoniae measurements available.

Laboratory methods

Laboratory workers unaware of the disease status of the participants analysed blood samples for C pneumoniae using whole organism antigen and time resolved fluorimetry.[6] The assay showed good agreement with microimmunofluorescence in a validation study of 480 people (intra-assay and interassay coefficients of variation were 4% and 8%). Serum lipid concentration, albumin concentration, leucocyte count, and packed cell volume were measured with standard assays, and C reactive protein and serum amyloid A concentrations were determined by sensitive enzyme immunoassays.[5]

Statistical methods and systematic review

We compared case and control groups using unmatched stratified logistic regression fitted by unconditional maximum likelihood (Stata Corporation, College Station, Texas, USA). Adjusted analyses included the following explanatory variables: age; cigarette smoking habit (never, former, current); daily cigarette consumption; non-fasting blood concentrations of total cholesterol, high density lipoprotein cholesterol, and triglyceride; markers of current social class (registrar general's 1980 classification with a separate category for armed forces); housing tenure (owner, private rent, council rent); marital status; current car ownership; father's occupation (manual, non-manual); and childhood social circumstances (father's occupation, family car ownership, bathroom in house, hot water tap in house, bedroom sharing). We prespecified analysis of C pneumoniae IgG titres by thirds of the values in controls--that is, the top third was defined as seropositive and the bottom third as seronegative. Previous systematic reviews suggested the need for adjustments for smoking and indicators of socioeconomic status in adulthood and childhood to help reduce any residual confounding in studies of coronary heart disease and persistent infective agents, and some previous studies of C pneumoniae infection and coronary heart disease have reported adjustments for indicators of social class both in adult life and in childhood (see Discussion).[2-5] We therefore prespecified that odds ratios would be reported both …