Overall eCO levels in asthmatics. The WMD of eCO levels between asthmatics and healthy subjects is 1.25 ppm (95%CI: 0.92 to 1.58) using the random effects model. Test for heterogeneity I
2 = 75%. SD: standard deviation; IV: inverse variance; CI: confidence interval; WMD: weighted mean difference; eCO: exhaled carbon monoxide.