Minimal clinically important difference in myasthenia gravis: outcomes from a randomized trial.

INTRODUCTION: The minimal clinically important difference (MCID) is the smallest outcome change that has clinical significance. Its use has not been established in the study of myasthenia gravis (MG).

METHODS: Patients from a published intravenous immunoglobulin (IVIg) vs. placebo study were studied. One anchor-based and 3 distribution-based techniques were used to identify quantitative myasthenia gravis score (QMGS), repetitive nerve stimulation (RNS), and single-fiber electromyography (SFEMG) MCID cut-offs. Patients with a change-score exceeding MCID cut-offs were compared.

RESULTS: MCID cut-offs were below a QMGS change of 3.0. Anchor-based and 1 × SEM cut-offs showed 58.3% vs. 30.7% responders (P = 0.017), ½ SD 54.2% vs. 19.2% responders (P = 0.018), and effect size 0.519 vs. 0.164 (P = 0.011) in IVIg vs. placebo. Anchor-based (P = 0.73) and effect-size (P = 0.41) MCID cut-offs did not show a difference between IVIg and placebo. MCID methods did not produce meaningful RNS cut-offs.

CONCLUSIONS: QMGS MCID values provide clinically relevant information and are recommended in MG trials. MCID analysis shows that improvement in MG patients treated with IVIg reflects clinically meaningful changes.