Objective: There are few lung function tests available to evaluate bronchial asthma in infants and toddlers. The objective of this study was to test the hypothesis that the measurement of exhaled carbon monoxide (eCO) levels is applicable to evaluate infants and toddlers with stable asthma and during acute asthma attack.

Methods: A one-way valve breath sampling bag was developed to collect the exhaled air of infants and toddlers. A total of 483 infants (under 2 years) and toddlers (2-5 years) were studied; 355 had an established diagnosis of asthma (182 suffering mild asthma attacks and 173 without active asthmatic symptoms), 119 had upper respiratory infection (URI) including acute bronchitis, and 9 were healthy.

Results: In infants and toddlers, eCO levels of those with asthma attacks [median (interquartile range) = 2.0 (2.0-3.25) ppm, n=182] were significantly higher than those of subjects with asymptomatic asthma [2.0 (1.0-2.0) ppm, n=173, P < 0.0001], URI [2.0 (1.0-3.0) ppm, n=119, P < 0.0001], and healthy children [1.0 (0.0-1.0) ppm, n=9, P < 0.0001]. In 75 children with asthma petit mal, eCO levels during asthma attacks [3.0 (2.0-4.0) ppm] significantly decreased after therapy [1.0 (1.0-2.0) ppm, P < 0.0001]. In infants and toddlers with an established diagnosis of asthma (n=355), eCO cut-off >2 ppm discriminated asthma attack from an asymptomatic state with a sensitivity of 95.6%, a specificity of 43.3%, and an area under the curve (AUC) of 0.71 (95% CI:0.65-0.76, P < 0.0001). In 401 infants and toddlers with some respiratory symptoms, of which 285 cases were finally diagnosed as asthma [eCO level = 2.0 (2.0-3.0) ppm] and 116 cases were not asthma [eCO level = 2.0 (1.0-3.0) ppm, P < 0.0001], eCO cut-off >3 ppm supported the final diagnosis of asthma with a sensitivity of 38.9%, a specificity of 74.1%, and AUC of 0.63 (95% CI:0.56-0.69, P < 0.0001).

Conclusion: The measurement of eCO by a novel method is applicable to evaluate asthmatic activity and treatment responsiveness, and to diagnose asthma in infants and toddlers.