Background. Several guidelines recommend that spirometry (FEV₁) should be measured as part of ongoing asthma management. Treatment decisions are made in response to a change in FEV₁; how much of a change is clinically meaningful is not well understood.

Methods.  Data were collated from five clinical trials where FEV₁ was measured at three-month intervals over a year. Between-visit changes were expressed as: %FEV₁; FEV₁ z score; and FEV₁ change score (Zc).  Zc was derived from clinically stable measurements in this population and adjusts z score for the child?s age and interval between measurements. The association between change in FEV₁ between visits and an asthma exacerbation within 3 months was estimated using logistic regression. 

Results. There were 8895 paired FEV₁ measurements from 1264 children. A relative decline in %FEV₁ between 14 and 20% was associated with an increased odds of asthma exacerbation within 3 months (OR range 1.5-1.9).  An absolute change in zFEV₁ was not associated with asthma exacerbations. An FEV₁ change score decline between 1.3 and 2.0 Zc was associated increased odds of exacerbation (OR range 1.3-1.6). Unexpectedly, rises of between 14 and 20 %FEV₁, 1.3-1.6 z score and 1.2-2.0 Zc were also associated with increased OR for exacerbation of 1.3-1.7. The mean initial FEV₁ z score for those with at least one Zc ?±1.6 during follow up was reduced by 0.4 [95% CI 1.8, 6.9] compared to those with Zc<±1.6 throughout.

Conclusion. A fall and a rise in % FEV₁ or change score was associated with a modest increase in the odds of a future exacerbation suggesting that both low ?baseline? FEV₁ and change in FEV₁ between visits can identify children at risk for asthma exacerbations.