Design
This study was designed as a reanalysis of collected data for the 30-item DASH questionnaire, from which scores for both the DASH and the Quick DASH were calculated. The data collection process for the assessment of the longitudinal construct validity of the DASH has been described previously [10]. The study was conducted in agreement with the local ethical guidelines for clinical studies and informed consent was obtained from the participants.
Questionnaire
The DASH questionnaire mainly consists of a 30-item disability/symptom scale. The two optional scales of the DASH (sport/music and work) were not part of the study. Each item in the disability/symptom scale has 5 response options. If at least 27 of the 30 items are completed a scale score, ranging from 0 (no disability) to 100 (most severe disability), can be calculated.
From the full-length DASH the 11 items that constitute the Quick DASH were extracted. To calculate a Quick DASH score at least 10 of the 11 items must be completed. Similar to the DASH, each item has 5 response options and, from the item scores, scale scores are calculated, ranging from 0 (no disability) to 100 (most severe disability).
The follow-up questionnaire included an item inquiring about change in the status of the arm as compared to its status before surgery. The item had 5 response options; much better, somewhat better, unchanged, somewhat worse, much worse. This item was accidentally missing in the initially mailed questionnaires and was therefore only completed by the last 83 participants, 82 of whom had Quick DASH scores and could be included in the present analysis.
Setting and participants
From an orthopedic department 109 of 118 consecutive patients with upper extremity disorders who fulfilled the eligibility criteria (scheduled for elective surgery, 18 years or older, symptom duration of at least 2 months, able to answer questionnaires) responded to the Swedish version of the DASH before surgery and at the follow-up evaluation. The follow-up was done at 6 to 21 (mean 12) months after surgery.
Of the 109 responders, 105 had responded to at least 10 of the 11 items used in the Quick DASH and were included in the analysis. The mean age of the 105 participants was 52 (range 18–83) years; 60 (57%) were women and 45 were men.
Analysis
The baseline, follow-up and change scores for the DASH and the Quick DASH were calculated for the whole population and for specific diagnostic groups.
To study the longitudinal construct validity the effect size (mean change score divided by the standard deviation of the baseline scores) and the standardized response mean (mean change score divided by the standard deviation of the change scores) for the DASH and Quick DASH were calculated.
To compare the performance of the DASH and the Quick DASH in discriminating among patients who differed in the degree of arm-related disability, receiver operating characteristic (ROC) curves were constructed using change scores (baseline to follow-up) as the test variable and patients' responses to the global item concerning perceived change in arm status after surgery as the dichotomized classifying variable; the difference in the areas under the ROC curves for the two questionnaire versions was calculated [11, 12]. In the first ROC analysis the DASH and Quick DASH were compared with regard to their ability to discriminate the patients who rated their arm status as "much better" or "somewhat better" (combined into one group) from those who rated it as "unchanged". In the second analysis the ability to discriminate the "much better" group from the "somewhat better" group was compared. The difference in the areas under the ROC curves indicates the magnitude of the difference in the discriminant ability of the two measures. The number of patients who had reported worsening was too small to perform an analysis comparing the ability of the 2 measures to detect deterioration.
To assess reliability the Cronbach alpha coefficient was calculated for the baseline and follow-up item responses. Agreement between the Quick DASH and the full-length DASH was assessed with the intraclass correlation coefficient (ICC) using the 2-way mixed and absolute agreement model [13]. The difference between the DASH scores and the Quick DASH scores was assessed with the paired-samples t-test. Because the Quick DASH responses were extracted from the full-length DASH some degree of correlation between part of the questionnaire and the whole is expected. To explore the possible effect of this factor we created two hypothetical 11-item short-forms by computer-generated random selection from the 30 items of the full-length DASH. These random 11-item short-forms were analyzed with regard to reliability in a similar fashion as done with the Quick DASH.
Test-retest reliability was studied in a subgroup of 30 patients (14 women) with a mean age of 54 (range 27–79) years, who had completed the full-length DASH on two occasions prior to surgery with a median interval of 5 (range 5–17) days [14]. The scores for the DASH, Quick DASH and the random short-forms from both response times were calculated. The ICC (2-way mixed, absolute agreement) and the paired-samples t-test were used for this analysis.