Background
The assessment process in both the clinical and research setting has progressively incorporated patient-reported outcome (PRO) measures and upper limb assessment is no exception. Regional and condition specific PROs enable the quantification of patient impairment [
1]. Doward and McKenna have described this as a '...needs based approach' [
2]. This assists the clinical decision-making process [
3,
4] and facilitates compliance with the protocols within professional organizations [
5], government agencies [
6,
7] and insurer groups [
8]. There are limited upper limb PROs developed specifically for the region as a single kinetic chain [
9] that accommodate the requirements of both the clinician and researcher in an efficient and effective manner [
10,
11].
The 30-item Disabilities Arm Shoulder and Hand (DASH) [
9] is reported to fulfil these criteria. It was validated for a variety of disorders [
1,
9,
12‐
16] and its availability in different languages has increased rapidly [
17‐
19]. The shorter 11-item QuickDASH was developed to reduce respondent and administrative burden and eliminate item redundancy. This improved compliance [
20], item redundancy and scale width for higher impairment conditions [
21]. Consequently, there is an impetus for the QuickDASH to replace the DASH [
22] and be advocated as a criterion standard for upper limb measurement [
23,
24]. However, the validity of the QuickDASH has been questioned as a consequence of conflicting findings on the factor structure [
25,
26]. A single factor structure is an essential property of all PROs that provide a single summated score [
27]. A PRO must exhibit a single predominant theme or factor, such as upper limb function, that is common to all item-questions. The factor structure must be unidimensional when analyzed. The most appropriate method is Maximum Likelihood extraction (MLE) [
28].
A literature search (PubMed, Medline, CINAHL, Embase, Cochrane and Google Scholar) found five prospective studies that investigated the QuickDASH. They considered the psychometric and practical characteristics in general populations [
24,
25,
29], burns patients [
30] and as a work injury prediction tool [
31]. The original validation [
20] and several subsequent studies reanalyzed data with the eleven items extracted from existing 30-item DASH responses [
21,
26,
32]. Only two studies investigated the QuickDASH factor structure. There was a unidimensional structure in the prospective study on the Japanese-language version [
25] but a bidimensional structure in reanalyzed extracted data of the French-language version [
26]. Both authors used principal component analysis which is considered inappropriate for PROs [
28]. Factor structure in the English-language version has not been reported. Consequently, the factor structure must be clarified and determined prospectively with appropriate item-extraction methodology in a general upper limb population.
The primary aim of this study was to determine the factor structure of the QuickDASH and QuickDASH-9. If unidimensional and valid, the next step was to calibrate and validate the psychometric properties and practical characteristics in independent general upper limb populations. Finally these characteristics were compared and correlated with the original full-length DASH and a validated criterion standard, the Upper Limb Functional Index (ULFI) [
11,
33].
Discussion
This study proposes the QuickDASH-9, with its valid unidimensional structure, as a way to overcome the existing shortcomings of the QuickDASH. This will enable the concept to continue. The modifications that produce the QuickDASH-9 fulfil the original aims of the QuickDASH [
20]: a shortened version of the full-length DASH with comparable or preferable psychometric properties, improved practicality and the elimination of item redundancy [
9,
11]. In attempting to achieve these aims the QuickDASH produced a bidimensional factor structure. Its validity as a single summated score cannot be supported.
Our findings propose the DASH scoring scale of 1-5 be modified to 0-4 in the QuickDASH-9. This uses the established format of a 0 based anchor rather than a 1 [
51]. This should facilitate practicality and ensure consistency of scoring with other PROs.
The bidimensional structure of the QuickDASH, demonstrated in this study using MLE, is consistent with previous findings by Fayad [
26] but conflicts with the unidimensional structure found by Imaeda [
25]. However, both previous researchers used principal component analysis which is not recommended [
28]. In this study the QuickDASH bidimensional structure demonstrated two factors that can be broadly divided into 'activity' and 'non-activity' items which supports previous findings [
21,
26]. The original DASH has a unidimensional structure [
33,
52‐
54]. This means the reductive process of concept-retention methodology, that reduces the DASH's 30 items to eleven in the QuickDASH, causes a fundamental change in the factor structure [
55]. It is critical that a PRO exhibits a unidimensional structure if it is to accurately reflect the measured region with a single summated score [
27].
There is a distinct lack of prospective studies of the QuickDASH and no English versions were found that investigated factor structure. Furthermore, reporting of psychometric properties is incomplete if the factor structure is not stated [
24,
29‐
31], and consequently misleading and the results invalid if the structure is not unidimensional.
The use of extracted items from the DASH as the sole method to validate the QuickDASH without prospective testing [
20,
21,
26,
32], should only be investigatory. This methodology risks shared measurement error and does not account for part or whole correlation [
56] which can lead to type I errors [
43]. By completing the prospective aspect of this study on a general upper limb population with a consistent regional reference standard, the ULFI, these error concerns are alleviated for the QuickDASH. However, for the QuickDASH-9 the same criticism applies as it is investigatory research only.
Should the findings of this study be supported by further research, then the QuickDASH-9 would be appropriate to replace the QuickDASH and also the original DASH. Similar proposals are already in place in other body regions. The Neck Disability Index, an advocated PRO, was recently shown to be invalid due to its bidimensional structure [
57,
58]. It is proposed that a shortened unidimensional version, the NDI-8 replaces the original [
58].
The reliability and responsiveness are lower in the QuickDASH-9 compared to the DASH. This is anticipated and consistent with previous QuickDASH findings [
20,
21,
26,
32] as the reduction in items from 30 to nine is substantial.
The QuickDASH-9 mean percentage scores were found to be higher than those of the DASH. This supports previous findings that a shortened tool with improved internal consistency will show greater scale width, particularly for higher impairment conditions [
21]. The choice of eleven items for the QuickDASH is based on the
a-priori assumption drawn from the 'Spearman-Brown prophesy'. Specifically, that a minimum of eleven items is required to produce an internal consistency within the clinically accepted range of 0.90 to 0.95 [
20]. This study has shown that in a shortened 9-item version, the internal consistency can remain within this range and provide a valid instrument with significant gains in practicality. However, a computational scoring aid is still required.
In both stages of this study the QuickDASH-9 showed inferior psychometric properties to the DASH and ULFI, particularly for reliability and error scores. In relation to the DASH this is outweighed by the gains in practicality and internal consistency, but not in comparison to the ULFI. These findings are reflected in the summary scores of the 'Measurement of Outcome Measures' and the 'Bot scale' that supports the preference of the QuickDASH-9 over the DASH. However, both tools remained notably lower than the ULFI on both scales which scores as the preferred instrument for both clinical and research purposes due to its practicality and lower missing responses.
Limitations
The study investigated only outpatients presenting to primary care physical therapy practices and further research is required to clarify these findings in an inpatient setting. The findings are general and extrapolation to specific conditions must be made with caution till such conditions are individually investigated. There was a consistent difference in the QuickDASH-9 order of factor loading between the calibration and validation stages. This is most likely from differences in the samples due to the diverse range of diagnoses and duration times used in each stage.
Strengths
The findings have broad implications for use in the general population as they are not specific to one condition or population group as participants were from general outpatient populations. Two independent population samples are used for data extraction to examine the QuickDASH-9 characteristics. The use of a consistent reference criterion, the ULFI, supports the similarity of findings in the two samples.
Implications for Practice
The QuickDASH-9 as a valid shortened form of the DASH provides a practical approach to measurement of the upper limb. This enhanced practicality reduces the burden to both the patient and clinician, optimizing clinical practice without compromising the accuracy and error measurement capacity of the instrument.
Implications for Research
A prospective validation of the QuickDASH-9 is required in an independent sample using an established criterion, such as the ULFI. Further investigation of the psychometric properties in samples of specific populations and conditions is also required. This could initially be investigative through extraction of responses from existing DASH and QuickDASH studies, with prospective investigation to follow. However, with the summary performance of all forms of the DASH concept shown to be lower than the ULFI, the adoption of the ULFI as a single preferred standard may be preferable.
Conclusions
The unidimensional structure found in the proposed QuickDASH-9 is valid and consistent with the full-length DASH. This achieves the original aim of the QuickDASH, to be a shortened version with comparable or preferable psychometric properties, no item redundancy and higher practicality. The QuickDASH, with a bidimensional structure, is invalid for the production of a summated score. This shortcoming is overcome by the QuickDASH-9. Furthermore, the QuickDASH-9 eliminates item redundancy found in the DASH, improves internal consistency, completion and scoring times and enhances practicality. The QuickDASH-9 offers a viable future option for the DASH concept.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
CPG is the principal investigator. He designed the study and is responsible for the protocol. CPG is also responsible for data acquisition and analysis. Together with CPG, BB and MY developed the key ideas underlying this study, interpreted the data, wrote and revised the manuscript. MM has been involved in interpretation of the data and revising the manuscript critically for shortcomings of the original QuickDASH and for the validation of the QuickDASH-9. All authors read and approved the final manuscript.