Rasch analysis of the Patient and Observer Scar Assessment Scale (POSAS) in burn scars
Martijn B. A. van der Wal
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Wim E. Tuinebreijer
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Monica C. T. Bloemen
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Pauline D. H. M. Verhaegen
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Esther Middelkoop
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Paul P. M. van Zuijlen
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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
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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
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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
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P. D. H. M. Verhaegen P. P. M. van Zuijlen (&) Department of Plastic
, Reconstructive,
and Hand Surgery, Red Cross Hospital
, Beverwijk,
The Netherlands
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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.
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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)