Rasch analysis of the Patient and Observer Scar Assessment Scale (POSAS) in burn scars

Quality of Life Research, Feb 2012

Purpose The Patient and Observer Scar Assessment Scale (POSAS) is a questionnaire that was developed to assess scar quality. It consists of two separate six-item scales (Observer Scale and Patient Scale), both of which are scored on a 10-point rating scale. After many years of experience with this scale in burn scar assessment, it is appropriate to examine its psychometric properties using Rasch analysis. Methods Cross-sectional data collection from seven clinical trials resulted in a data set of 1,629 observer scores and 1,427 patient scores of burn scars. We examined the person–item map, item fit statistics, reliability, response category ordering, and dimensionality of the POSAS. Results The POSAS showed an adequate fit to the Rasch model, except for the item surface area. Person reliability of the Observer Scale and Patient Scale was 0.82 and 0.77, respectively. Dimensionality analysis revealed that the unexplained variance by the first contrast of both scales was 1.7 units. Spearman correlation between the Observer Scale Rasch measure and the overall opinion of the clinician was 0.75. Conclusion The Rasch model demonstrated that the POSAS is a reliable and valid scale that measures the single-construct scar quality.

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Rasch analysis of the Patient and Observer Scar Assessment Scale (POSAS) in burn scars

Martijn B. A. van der Wal 0 1 2 3 4 Wim E. Tuinebreijer 0 1 2 3 4 Monica C. T. Bloemen 0 1 2 3 4 Pauline D. H. M. Verhaegen 0 1 2 3 4 Esther Middelkoop 0 1 2 3 4 Paul P. M. van Zuijlen 0 1 2 3 4 0 M. B. A. van der Wal P. D. H. M. Verhaegen E. Middelkoop P. P. M. van Zuijlen Department of Plastic , Reconstructive, and Hand Surgery, VU medical Center , Amsterdam, The Netherlands 1 M. B. A. van der Wal M. C. T. Bloemen P. D. H. M. Verhaegen E. Middelkoop P. P. M. van Zuijlen Burn Center, Red Cross Hospital , Beverwijk, The Netherlands 2 M. B. A. van der Wal W. E. Tuinebreijer M. C. T. Bloemen P. D. H. M. Verhaegen E. Middelkoop P. P. M. van Zuijlen Association of Dutch Burn Centers , Beverwijk, The Netherlands 3 P. D. H. M. Verhaegen P. P. M. van Zuijlen (&) Department of Plastic , Reconstructive, and Hand Surgery, Red Cross Hospital , Beverwijk, The Netherlands 4 P. D. H. M. Verhaegen P. P. M. van Zuijlen Department of Plastic , Reconstructive, and Hand Surgery, Academic Medical Center , Amsterdam, The Netherlands Purpose The Patient and Observer Scar Assessment Scale (POSAS) is a questionnaire that was developed to assess scar quality. It consists of two separate six-item scales (Observer Scale and Patient Scale), both of which are scored on a 10-point rating scale. After many years of experience with this scale in burn scar assessment, it is appropriate to examine its psychometric properties using Rasch analysis. Methods Cross-sectional data collection from seven clinical trials resulted in a data set of 1,629 observer scores and 1,427 patient scores of burn scars. We examined the person-item map, item fit statistics, reliability, response category ordering, and dimensionality of the POSAS. - Abbreviations POSAS Patient and Observer Scar Assessment Scale VSS Vancouver Scar Scale MNSQ Mean Square Burn scars are known for their impact on the quality of life due to an array of functional, cosmetic, and psychological problems, related to scarring [13]. Several appropriate instruments are available that have been tested and validated to evaluate scar quality [46]. Scar assessment scales are often used because they are easily accessible and free of charge [7, 8]. In 2004, the Patient and Observer Scar Assessment Scale (POSAS) was introduced [9], which aimed at measuring the quality of scar tissue. The POSAS consists of an Observer and a Patient Scale and includes a comprehensive list of items, based on clinically relevant scar characteristics [10]. The observer scores six items: vascularization, pigmentation, thickness, surface roughness, pliability, and surface area. The patient scores six items: pain, pruritus, color, thickness, relief, and pliability (see Appendix) [10]. All included items are scored on the same polytomous 10-point scale, in which a score of 1 is given when the scar characteristic is comparable to normal skin and a score of 10 reflects the worst imaginable scar. All items are summed to give a total scar score, and therefore, a higher score represents a poorer scar quality. Studies that compared the POSAS with the widely used Vancouver Scar Scale revealed that the former was more reliable than the latter [9, 11]. At present, the POSAS is being used to evaluate the rehabilitation process in different types of injury [1119] and has been advocated by many for scar assessment [2, 8, 11, 20]. Currently, all available scar assessment scales, including the POSAS, have been constructed and tested following principles of the classical test theory (CTT). However, modern test theories are considered superior to the CTT as it makes stronger assumptions and provides stronger findings. For this reason, the Rasch measurement model, one of the item response theory (IRT) models, is nowadays frequently applied in quality-of-life research [2126]. Use of Rasch methodology involves a rigorous and extensive analysis of the data and provides additional psychometric information that cannot be obtained through the CTT approach. The data are tested for fit into the Rasch model, allowing for a detailed examination of the internal construct validity of the scale, including properties such as reliability and ordering of the categories. It also determines whether a scale is unidimensional, which is required to justify summation of scores and can linearly transform raw scores from their original scale to an interval scale to allow application of parametric statistics. After several years of using the POSAS for burn scar evaluation, it became appropriate to subject this tool to modern test theories. For this reason, we decided to apply the Rasch model [27] to our data. Materials and methods Data collection Observer and Patient Scale scores were collected from a large database including five single-center and two multicenter clinical trials involving burn scars. All scores were obtained by clinical evaluation of the scars. In these trials, the scars were usually scored by multiple observers and also on multiple time (...truncated)


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Martijn B. A. van der Wal, Wim E. Tuinebreijer, Monica C. T. Bloemen, Pauline D. H. M. Verhaegen, Esther Middelkoop, Paul P. M. van Zuijlen. Rasch analysis of the Patient and Observer Scar Assessment Scale (POSAS) in burn scars, Quality of Life Research, 2012, pp. 13-23, Volume 21, Issue 1, DOI: 10.1007/s11136-011-9924-5