title: Psychometric Properties of the Quantitative Myasthenia Gravis Score and the Myasthenia Gravis Composite Scale.
abstract: BACKGROUND
  The Quantitative Myasthenia Gravis Score and the Myasthenia Gravis Composite are two commonly used outcome measures in Myasthenia Gravis. So far, their measurement properties have not been compared, so we aimed to study their psychometric properties using the Rasch model.
  
  
  METHODS
  251 patients with stable myasthenia gravis were assessed with both scales, and 211 patients returned for a second assessment. We studied fit to the Rasch model at the first visit, and compared item fit, thresholds, differential item functioning, local dependence, person separation index, and tests for unidimensionality. We also assessed test-retest reliability and estimated the Minimal Detectable Change.
  
  
  RESULTS
  Neither scale fit the Rasch model (X2p <  0.05). The Myasthenia Gravis Composite had lower discrimination properties than the Quantitative Myasthenia Gravis Scale (Person Separation Index: 0.14 and 0.7). There was local dependence in both scales, as well as differential item functioning for ocular and generalized disease. Disordered thresholds were found in 6(60%) items of the Myasthenia Gravis Composite and in 4(31%) of the Quantitative Myasthenia Gravis Score. Both tools had adequate test-retest reliability (ICCs >0.8). The minimally detectable change was 4.9 points for the Myasthenia Gravis Composite and 4.3 points for the Quantitative Myasthenia Gravis Score.
  
  
  CONCLUSIONS
  Neither scale fulfilled Rasch model expectations. The Quantitative Myasthenia Gravis Score has higher discrimination than the Myasthenia Gravis Composite. Both tools have items with disordered thresholds, differential item functioning and local dependency. There was evidence of multidimensionality in the QMGS. The minimal detectable change values are higher than previous studies on the minimal significant change. These findings might inform future modifications of these tools.
authors.affiliation: Division of Neurology - Department of Medicine. University Health Network, Toronto General Hospital, Toronto, Canada.
authors.forename: Carolina
authors.initials: C
authors.lastname: Barnett
authors.affiliation: Department of Neurology, Maastricht University Medical Centre, Maastricht, The Netherlands.|Spaarne Hospital, Hoofddorp, The Netherlands.
authors.forename: Ingemar S J
authors.initials: IS
authors.lastname: Merkies
authors.affiliation: Division of Neurology - Department of Medicine. University Health Network, Toronto General Hospital, Toronto, Canada.
authors.forename: Hans
authors.initials: H
authors.lastname: Katzberg
authors.affiliation: Division of Neurology - Department of Medicine. University Health Network, Toronto General Hospital, Toronto, Canada.
authors.forename: Vera
authors.initials: V
authors.lastname: Bril
keywords: MGC; Myasthenia gravis; QMGS; Rasch; outcome assessment (Health Care)