Longitudinal co-variations between inflammatory cytokines, lung function and patient reported outcomes in patients with asthma.

BACKGROUND: Asthma is a chronic inflammatory respiratory disorder associated with reduced lung function and poor quality of life. The condition is also associated with poor self-rated health, a major predictor of objective health trajectories. Of biological correlates to self-rated health, evidence suggests a role for inflammatory cytokines and related sickness behaviours. However, this is mainly based on cross-sectional data, and the relation has not been investigated in patients with chronic inflammatory conditions.

OBJECTIVE: To investigate inflammatory cytokines, lung function, sickness behaviour and asthma-related quality of life as determinants of self-rated health in patients with asthma, and to investigate if these variables co-vary over time.

METHODS: Plasma cytokines (IL-5, IL-6), lung function (FEV1), sickness behaviour, asthma-related quality of life and self-rated health were assessed in 181 patients with allergic asthma aged 18-64 years in a one-year longitudinal study. Mixed effect regression models and Spearman's correlation were performed to analyse the associations between repeated measurements.

RESULTS: More sickness behaviour and poorer asthma-related quality of life were associated with poorer self-rated health (p's<0.001). In men, both low and high levels of interleukin (IL)-6 and poorer lung function were related with poorer self-rated health (p's<0.05). Over the year, improved asthma-related quality of life was associated with better self-rated health (Spearman's rho = -0.34 women,-0.36 men, p's<0.01). Further, if sickness behaviour decreased, self-rated health improved, but only in women (Rho = -0.21, p<0.05). Increased FEV1 in men was associated with an increase in IL-6 (Rho = 0.24, p<0.05) as well as improved self-rated health (Rho = -0.21, p<0.05) and asthma-related quality of life (Rho = 0.29, p<0.01) over the year.

CONCLUSION: The study highlights the importance of subjectively perceived sickness behaviour and asthma-related quality of life together with lung function as determinants of self-rated health in asthmatic patients. The importance of inflammatory activation for patient reported outcomes in chronic inflammatory conditions need further investigation.