Research article

Longitudinal proxy measurements in multiple sclerosis: patient-proxy agreement on the impact of MS on daily life over a period of two years

Femke AH van der Linden^{* 1,2} , Jolijn J Kragt¹ , Margarethe van Bon¹ , Martin Klein² , Alan J Thompson⁴ , Henk M van der Ploeg² , Chris H Polman¹ and Bernard MJ Uitdehaag^{* 1,3}

¹Department of Neurology, VU University Medical Centre, Amsterdam, The Netherlands

²Medical Psychology, VU University Medical Centre, Amsterdam, The Netherlands

³Clinical Epidemiology and Biostatistics, VU University Medical Centre, Amsterdam, The Netherlands

⁴Neurological Outcome Measures Unit, Institute of Neurology, London, UK

author email corresponding author email* Contributed equally

BMC Neurology 2008, 8:2doi:10.1186/1471-2377-8-2

Published:	28 February 2008

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.

Conclusion

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.