title: Longitudinal proxy measurements in multiple sclerosis: patient-proxy agreement on the impact of MS on daily life over a period of two years.
abstract: BACKGROUND
  The use of self-report measurements in clinical settings is increasing. However, in patients with limitations that interfere with reliable self-assessment such as cognitive impairment or mood disturbances, as may be the case in multiple sclerosis (MS), data collection might be problematic. In these situations, information obtained from proxy respondents (e.g. partners) may replace self-ratings. The aim of this study was to examine the value of proxy ratings at separate points in time and to assess patient-proxy agreement on possible changes in disease impact of MS.
  
  
  METHODS
  Fifty-six MS patients and their partners completed the Multiple Sclerosis Impact Scale (MSIS-29) at baseline and follow-up, two years later. Patient-proxy agreement was assessed at both time points by calculating intraclass correlation coefficients (ICCs), exact and global agreement and the mean directional differences between groups. Agreement of change over time was assessed by calculating ICCs between change scores. In parallel, global ratings of both patients and proxy respondents of the extent to which the patient had improved or deteriorated over the past two years were collected to validate possible changes on the MSIS-29.
  
  
  RESULTS
  At both time points, agreement on the physical scale was higher than agreement on the psychological scale (ICCs at baseline were 0.81 for the physical scale and 0.72 for the psychological scale; at follow-up, the ICC values were 0.86 and 0.65 respectively). At follow-up, statistically significant mean differences between patients and proxies were noted for the physical scale (-4.8 +/- 12.7, p = 0.006) and the psychological scale (-8.9 +/- 18.8, p = 0.001). Agreement between change scores on the MSIS-29 was fair (ICC < 0.60). Our analyses suggest that the validity of measuring changes over time might be better for proxy respondents compared to patients.
  
  
  CONCLUSIONS
  Proxy respondents could act as a reliable source of information in cross-sectional studies. Moreover, results suggested that agreement on change over time might be better for proxy respondents compared to patients. Although this remarkable finding should be interpreted cautiously because of several limitations of the study, it does plead for further investigation of this important topic.
authors.affiliation: Department of Neurology, VU University Medical Centre, Amsterdam, The Netherlands. fah.vdlinden@vumc.nl
authors.forename: Femke A H
authors.initials: FA
authors.lastname: van der Linden
authors.forename: Jolijn J
authors.initials: JJ
authors.lastname: Kragt
authors.forename: Margarethe
authors.initials: M
authors.lastname: van Bon
authors.forename: Martin
authors.initials: M
authors.lastname: Klein
authors.forename: Alan J
authors.initials: AJ
authors.lastname: Thompson
authors.forename: Henk M
authors.initials: HM
authors.lastname: van der Ploeg
authors.forename: Chris H
authors.initials: CH
authors.lastname: Polman
authors.forename: Bernard M J
authors.initials: BM
authors.lastname: Uitdehaag
mesh_terms: D000203:Activities of Daily Living / Q000523:psychology*; D000328:Adult; D016022:Case-Control Studies; D015331:Cohort Studies; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D009103:Multiple Sclerosis / Q000453:epidemiology* / Q000523:psychology*; D011446:Prospective Studies; D019727:Proxy / Q000523:psychology*; D012107:Research Design / Q000639:trends*; D012720:Severity of Illness Index