Discrimination ratio analysis of inflammatory markers: implications for the study of inflammation in chronic disease

Abstract

To understand the role of inflammation in chronic disease it is important to have a reliable measure of habitual inflammatory status. A number of acute-phase response markers have been used as measures of inflammatory status, but the ability of a single measure to appropriately reflect habitual inflammatory status has not been assessed. This study compares the ability of different inflammatory markers to characterize habitual inflammatory status in overweight women. A single fasting blood sample was taken from 86 overweight women (mean body mass index [BMI], 35.2 kg/m 2; range, 26.2 to 47.6 kg/m 2) and a number of inflammatory markers (both acute-phase response markers and cytokines) were measured. A randomly selected subpopulation of 15 women attended on 2 further occasions for further blood samples. Using the subpopulation, discrimination ratios (DRs) were calculated for each inflammatory marker to assess the within-subject variability. The DRs were then used to determine the relationship between these markers, adjusted for within-subject variability, in the whole population. In this highly controlled experimental environment, interleukin-6 (IL-6), with a DR of 3.71, was the cytokine with the greatest ability to discriminate between subjects, suggesting that it is best able to characterize habitual inflammatory status. Sialic acid was the acute-phase response marker with the highest DR (3.16), and showed stronger correlations with other inflammatory markers, including C-reactive protein (CRP), than IL-6. This study suggests that use of some inflammatory markers, such as CRP, with large within-individual variability, will underestimate the relationship between inflammation and disease, and thus relationships between inflammation and chronic disease may be stronger than previously appreciated. Future studies should consider IL-6 or sialic acid to provide a more robust measure of inflammatory status.