Lung function apparatus may be a source of cross infection in patients with cystic fibrosis and may be prevented using a 'bag in bottle' system. As this system has never been validated in children the flow volume loops performed using a Vitalograph Compact in the usual way were compared with results obtained blowing into the bag and bottle system, in random order, in 20 children with asthma (age 8-15 years). Three reproducible flow volume loops were recorded for each apparatus. There was no order effect. Mean (SE) results for forced expiratory volume in one second, forced vital capacity (FVC), peak expiratory flow rate, and maximum expiratory flow at between 75% and 25% of FVC showed respectively that the bag in bottle results were significantly greater than those obtained blowing directly into the spirometer by 90 (17) ml, 12.7 (5.1) l/min, 130 (60) ml/min. Although statistically significant, these differences are within the previously described coefficients of variation for children with cystic fibrosis and are of doubtful clinical relevance. It is concluded that the bag in bottle system does not introduce any important systematic error. It potentially offers absolute protection from cross infection.
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