A Meta-analysis of the Association of Exhaled Carbon Monoxide on Asthma and Allergic Rhinitis

The objective of this study is to evaluate the effect of exhaled CO (eCO) on the development of asthma and allergic rhinitis (AR) by means of reviewing published literature. The literatures published between January 1997 and December 2008 from the US National Library of Medicine (NLM) Database were obtained according to inclusion criteria. Meta-analysis of randomized controlled trials (RCTs) was performed. CO levels of asthma and AR patients were compared with that of normal controls. HO-1(heme oxygenase-1) expression and effect of corticosteroids on eCO levels were also analyzed. Fifteen studies concerning asthma and four studies concerning AR were included in this analysis. Heterogeneity from different studies was evident (P < 0.0001), so a random-effects model was preferred. The meta-analysis revealed that asthmatic patients had significantly higher levels of eCO compared to normal controls. There was significant difference between asthma and control groups in terms of eCO (combined weighted mean difference (WMD) 1.33 (95% confidence interval 0.72 to 1.95), P < 0.0001), and no significant difference between AR and control (combined WMD 0.93 (95% confidence interval -0.54 to 2.40), P = 0.22). HO-1 expression were also reviewed, asthma group produced greater expression of HO-1 than control group with significant difference (combined standardized mean difference (SMD) 2.98 (95% confidence interval 1.13 to 4.84), P = 0.002). After corticosteroid therapy, significantly different levels of eCO were produced after corticosteroid therapy than did asthma group (combined WMD -1.23 (95% confidence interval -2.43 to -0.03), P = 0.04). The analysis reveals that eCO levels were significantly raised in asthma and it may attribute to high expression of HO-1, but there were no significantly high eCO levels between AR and control groups. Due to sensitivity to corticosteroid inhibition, eCO may be used as a practical marker to detect and monitor exacerbation of asthma.