We thank Durani for his comments and welcome his call to distinguish between the Patient-Reported Impact of Scars Measure1 and the Patient Scar Assessment Questionnaire.2 We agree that both studies and instruments have strengths and weaknesses, and appreciation of these will highlight where further work and collaboration may be useful. However, there are a number of assertions made by the author that require clarification.
The Patient Scar Assessment Questionnaire relied on classic test theory to establish its quality. With this approach, emphasis is placed on internal consistency, factor analysis, correlational analyses, and reliability. In recent years, classic test theory has been replaced by item response theory and, specifically, Rasch analysis in instrument development and evaluation. To achieve fundamental measurement, certain properties are required:
- The numerical properties of order (one mark on the scale represents more or less of the construct than another).
- Addition (points on scales may be added together).
- Specific objectivity (the calibration of the scale is independent of the persons used to calibrate and vice versa) must be met.
Where data fit the Rasch model, these properties are confirmed and fundamental measurement follows. Internal consistency and factor analysis do not determine whether or not scales are unidimensional. However, where data collected with a scale fit the Rasch model, unidimensionality is possible and can be assessed. In addition to allowing item scores to be added together, Rasch analysis has several additional benefits for instrument development:
- It guides item reduction by identifying items that misfit the model (i.e., measure a different construct from the other items), are redundant, or exhibit bias related to extraneous variables such as age or sex.
- It shows whether the chosen response format is working.
- Given that both patients and items are calibrated on the same underlying metric trait, it is possible to see whether the items in a scale are matched to the population being assessed. This allows end effects to be avoided.
Although no measurement model for the Patient Scar Assessment Questionnaire was provided by the authors, it is clear from its items that it predominantly measures impairment (symptoms). This reflects the reliance on literature and clinical experts in item generation. Consequently, the questionnaire is equivalent to the Patient-Reported Impact of Scars Measure symptom scale. Such a scale is useful in that it may provide information on outcomes of interest to the clinician.
However, patients may not be concerned by the “symptoms” per se but rather by how their life is affected by the symptoms they experience. This is the role of the Patient-Reported Impact of Scars Measure needs-based quality-of-life scale. Quality of life is the most important patient-based outcome, and its measurement is fundamental to determining whether or not patients benefit from interventions. The content of the Patient-Reported Impact of Scars Measure quality-of-life scale is derived entirely from patients—the only informants capable of commenting on the impact of scars on quality of life. Durani suggests that patients' perceptions of scar appearance are missing from the measure. However, the issue was identified in the measure's development process and is actually covered by several quality-of-life items.3
He acknowledges that the Patient Scar Assessment Questionnaire was tested with patients who overwhelmingly had mild scars and that Patient-Reported Impact of Scars Measure content and testing was conducted with patients with relatively severe scars. One of the key requirements of the Patient-Reported Impact of Scars Measure was that it should be relevant to patients who present to health services for treatment. It is unlikely that patients with very mild scars will have impaired quality of life and, consequently, will not seek treatment for them. It is difficult to conceive of a clinical trial that would only be interested in patients with mild scars. Such a trial would not convince payers of the need to prescribe the tested product—especially if quality of life was not measured.
Consideration of the classic test theory–based psychometric results for the Patient Scar Assessment Questionnaire suggests that it is in need of refinement. The test-retest reliability (responsiveness) results are almost all below a level required for instruments to be used on an individual basis. This finding indicates that the Patient Scar Assessment Questionnaire will produce high levels of measurement error with low levels of responsiveness. Furthermore, given that the measure assesses symptoms, the correlations observed with the Manchester Scale Score are surprisingly low. The only other test of validity in the study was correlation between the questionnaire and the visual analogue scale scores. However, the latter ranged from “excellent scar” to “poor scar,” and it is unclear what they actually measured.
Durani makes it clear that the Patient Scar Assessment Questionnaire is still in a development phase. We recommend that subsequent data collected with the measure be subjected to Rasch analysis in an attempt to see whether it is possible to identify a single working unidimensional symptoms scale from within its current items. The final scale would benefit from a simplified response format and from having sufficient items to cover the range of potential symptoms. Measures that consist of several subscales make interpretation difficult. For example, if some symptoms improve and others deteriorate with treatment, is the intervention of value?
There is, of course, additional work that can be performed to evaluate the value of the Patient-Reported Impact of Scars Measure. In particular, it is important to test its responsiveness, although measures with good reproducibility are likely to be responsive. If changes are made to the Patient Scar Assessment Questionnaire as outlined above, it would be helpful to compare it to the Patient-Reported Impact of Scars Measure symptoms scale to see which is more useful in measuring scar symptoms. However, the Patient-Reported Impact of Scars Measure quality-of-life scale remains the only measure available for assessing the impact of scars from the patient's perspective.
Benjamin C. Brown, M.B.Ch.B.
Plastic and Reconstructive Surgery Research
Manchester Interdisciplinary Biocenter
University of Manchester
Stephen P. McKenna, Ph.D., C.Psychol., A.F.B.Ps.S.
Galen Research Ltd.
Ardeshir Bayat, Ph.D., M.R.C.S.
Plastic and Reconstructive Surgery Research
Manchester Interdisciplinary Biocenter
University of Manchester, and
Manchester Academic Health Science Center
South Manchester University Hospital Foundation Trust
Manchester, United Kingdom
1. Brown BC, McKenna SP, Solomon M, Wilburn J, McGrouther DA, Bayat A. The patient-reported impact of scars measure: Development and validation. Plast Reconstr Surg
2. Durani P, McGrouther DA, Ferguson MW. The Patient Scar Assessment Questionnaire: A reliable and valid patient-reported outcomes measure for linear scars. Plast Reconstr Surg
3. Brown BC, McKenna SP, Siddhi K, McGrouther DA, Bayat A. The hidden cost of skin scars: Quality of life after skin scarring. J Plast Reconstr Aesthet Surg
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