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To the Editor:
The efficacy of interferon-beta (IFNβ) in secondary progressive multiple sclerosis (MS) is less clear than in relapsing remitting MS.1,2⇓ Therefore, it is not surprising that Polman et al. did not show an impact of neutralizing anti-IFNβ antibodies (NAB) on disease progression.3 This trial is underpowered to detect an effect of NAB on disease progression.3
In this study, 100 of 360 (28%) IFNβ-1b treated patients developed NAB.3 Applying the results of the study and assuming that NAB+ patients behave as if on placebo, one would expect at least 49.8% of the 100 NAB+ patients compared to a maximum of 38.9% of the 260 NAB− patients to progress over the 3 years of the study. At a level of significance of 0.029, which was used in the original study, a two-sided test would only have a 35% chance of detecting a significant difference between NAB+ and NAB− patients (Fisher exact test). This calculation overestimates the power of the study as it ignores the observed therapeutic effect of IFNβ-1b before the development of NAB.3
Polman et al. argue that a cross-sectional analysis is …
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