Factor analysis of risk variables associated with low-grade inflammation

Background: Chronic subclinical inflammation, manifesting as elevated levels of inflammatory markers such as C-reactive protein (CRP), predicts future atherothrombotic events. The pathophysiology of low-grade inflammation is complex, and multiple intercorrelated conditions have been associated with elevated CRP. Methods: Principal factor analysis was used to investigate clustering of variables associated with elevated CRP using data from 1435 subjects without known coronary disease. Components of the metabolic syndrome, uric acid, liver enzymes, pulmonary function tests, smoking status, cardiorespiratory fitness (measured by maximal treadmill test), and high-sensitivity C-reactive protein were determined in each subject. Results: Factor analysis identified three factors, which explained 51.0% of the total variance in the dataset (24.4% factor 1, 17.3% factor 2, and 9.3% factor 3). Based on factor loadings of ≥0.5, these factors were interpreted as (1) "metabolic factor" including BMI, fasting glucose, HDL cholesterol, triglycerides, systolic blood pressure, and uric acid; (2) a cardiorespiratory factor that included fitness level, forced expiratory volume in 1 s and sex; and (3) "smoking" factor that included cigarette smoking and age. Each of these factors was significantly associated with the presence of high-risk CRP (≥3 mg/L) in the study population. The ability of a multivariate model that included these three factors to predict high-risk CRP was comparable to a model containing the original 10 variables (area under the receiver-operator characteristics curve 0.7 vs. 0.72, respectively). Conclusion: Metabolic perturbations, cardiorespiratory fitness, and smoking are separate and largely independent factors in the pathophysiology of chronic, low-grade inflammation.