eCO levels in different asthma control states. The WMD of eCO levels between controlled + partly controlled asthmatics and healthy subjects is 1.20 ppm (95%CI: 0.39 to 2.01) using the random effects model. Test for heterogeneity I
2 = 91%. The WMD of eCO levels between uncontrolled asthmatics and healthy adults is 2.12 ppm (95%CI: 0.56 to 3.68) using the random effects model. Test for heterogeneity I2 = 95%. SD: standard deviation; IV: inverse variance; CI: confidence interval; WMD: weighted mean difference; eCO: exhaled carbon monoxide.