eCO levels in intermittent asthmatics. The WMD of eCO levels between intermittent asthmatics and healthy subjects is 0.31 ppm (95%CI: 0.16 to 0.46) using the fixed effects model. Test for heterogeneity I
2 = 11%. SD: standard deviation; IV: inverse variance; CI confidence interval; WMD: weighted mean difference; eCO: exhaled carbon monoxide.