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.