Related Experiment Video
Updated: May 7, 2026
Minimal Erythema Dose (MED) Testing
Published on: May 28, 2013
The minimally important difference of the Asthma Control Test
Michael Schatz1, Mark Kosinski, Aaron S Yarlas
1Department of Allergy, Kaiser Permanente Medical Center, San Diego, Calif. 92111, USA. michael.x.schatz@kp.org
Background:
The Asthma Control Test (ACT) has been well validated, but a minimally important difference (MID) has not been established.
Objective:
We sought to identify an MID for the ACT.
Methods:
Data come from 4 independent samples of adult asthmatic patients. Distributional methods for determining the MID included 0.5 SD, 1 SEM, and 2 SEM. Anchor-based methods assessed the relationship of differences in ACT scores to (1) self-reported asthma severity, (2) asthma episode frequency in the past 4 weeks, (3) physician ratings of asthma control, (4) physician recommendation of a change in therapy, (5) FEV(1), (6) the risk over the next 12 months of excess short-acting beta-agonist use and exacerbations, and (7) patient-defined changes in asthma course over 3 months.
Results:
Four thousand one hundred eighteen patients completed the ACT. The 0.5 SD criterion for MID ranged from 2.03 to 2.45 points (mean, 2.2 points). The 1 SEM criterion ranged from 1.77 to 2.05 points (mean, 1.88 points), and the 2 SEM criterion ranged from 3.55 to 4.10 points (mean, 3.75 points). Differences in mean ACT scores across patient groups differing on criterion measures ranged from 1.06 to 5.28 points (mean, 3.1 points). Predictive analyses showed that a difference of 3 points on the ACT was associated with a subsequent 76% increased risk (95% CI, 73% to 79%) of excess short-acting beta-agonist use and a 33% increased risk (95% CI, 31% to 35%) of exacerbations.
Conclusion:
The data support an MID for the ACT of 3 points.
Insights
A minimally important difference (MID) for the Asthma Control Test (ACT) was established. A 3-point change on the ACT indicates a meaningful improvement in asthma control for patients.
Area of Science:
- Pulmonary Medicine
- Clinical Assessment
- Asthma Research
Background:
- The Asthma Control Test (ACT) is a validated tool for assessing asthma control.
- A minimally important difference (MID) for the ACT has not been previously established.
Purpose of the Study:
- To determine the minimally important difference (MID) for the Asthma Control Test (ACT).
Main Methods:
- Utilized data from 4 independent adult asthma patient samples.
- Employed distributional methods (0.5 SD, 1 SEM, 2 SEM) and anchor-based methods.
- Anchor-based methods correlated ACT score changes with asthma severity, episode frequency, physician ratings, therapy changes, FEV1, medication use, and patient-defined outcomes.
Main Results:
- Distributional MID estimates ranged from 1.88 to 3.75 points.
- Anchor-based analyses showed mean ACT score differences between patient groups of 3.1 points.
- A 3-point ACT difference predicted a 76% increased risk of excess SABA use and a 33% increased risk of exacerbations.
Conclusions:
- The findings support a minimally important difference (MID) of 3 points for the Asthma Control Test (ACT).
- This 3-point MID provides a clinically relevant threshold for interpreting changes in asthma control.
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