The importance of monitoring the effectiveness of treatment is well recognized and furthermore is the foundation of evidence-based health care. For this purpose instruments that have the ability to detect changes and can differentiate a small difference from a large difference are needed. In a previous study, the DASH score change was reported for 172 patients with different upper extremity disorders (such as shoulder arthritis and carpal tunnel syndrome). The mean change between baseline and followup scores 12 weeks after treatment was 13 (SD 17), the effect size was 0.6 and the standardized response mean was 0.8. The changes were also shown for patients rating their problem as better (mean score change 17, effect size 0.75, standardized response mean 1.1) and patients rating their function as better (mean score change 20, effect size 0.8, standardized response mean 1.2). Also, based on the results of the present study, it appears that the DASH has the ability to detect changes on group level corresponding to the patients' perception after surgery in a variety of upper extremity disorders. A significant difference in DASH scores between patients responding "much better/worse" and "somewhat better/worse" was found showing the instruments ability to discriminate between these degrees of change. A mean score change of 19 indicated a change in disability rated as "much better/worse" and a mean score change of 10 as "somewhat better/worse". It has been suggested that the score change rated as "somewhat changed" could be defined as the limit for minimal important change. This information could then be used for power calculations when planning prospective studies. In a recent study a DASH score change of 15 has been suggested to discriminate between improved and unimproved patients. This was based on the patients' responses to a question about "being able to cope with the problem and do what you would like to do", with a response change from "not being able to cope" before treatment to "being able to cope" at followup considered as criterion for improvement. However, we believe that a change in disability can be important even if the patients are not able to do all what they want to do or, at a particular time, not being able to cope with the problem. Future investigations are needed to determine whether the DASH is sensitive to milder degrees of impact other than that of surgery.
The difference noted in the group stating no change (mean score change -0.3) can be seen as the difference that occurred by chance and was similar to the score change previously reported[3, 7]. A difference of this size should not be considered as a real change of upper extremity disability.
In the analysis of health transition only the last 83 patients were included because the item was accidentally missing in the initially mailed questionnaires. The mean change in DASH score did not significantly differ between the patients who did not receive and those who responded to the transition item suggesting that it is unlikely the missing item could have substantially influenced the results.
We chose to use self-rated change of health status in the operated arm as external criterion in order to ensure that it did not capture global health changes not related to the upper extremities.
The minimum followup time in the present study was 6 months and the latest response was received 21 months after surgery. The minimum followup time was chosen as it was expected to be sufficient to show improvement after surgery in most disorders. As shown in the correlation analysis time since surgery had, within this followup period, only weak-to-moderate but statistically non-significant association with the change in DASH scores after arthroscopic acromioplasty and carpal tunnel release. However, the difference in followup time is a limitation that can have implication, particularly when interpreting the size of change in DASH score for the assessment of treatment effectiveness. The possible implication of response shift also needs to be evaluated in future studies.
In this study the DASH demonstrated high Cronbach alpha values, indicating an excellent internal consistency that is adequate for group as well as for individual comparisons. These results support the use of the DASH to measure changes in upper extremity function also on an individual level. However, for individual patient assessment with the DASH the magnitude of score change has to be studied on individual level. It is important to note that in the present study only longitudinal construct validity on group level has been analyzed.
The treatment effectiveness calculations showed that for arthroscopic acromioplasty the effect size was larger than the standardized response mean, while for carpal tunnel release the opposite was shown. This illustrates the difficulties with interpretation of such calculations when only one of the analyses is presented. Since the effect size is dependent on the homogeneity of the group preoperatively and the standardized response mean is dependent on the homogeneity of the change of disability, these calculations will by nature differ in almost any group. Both calculation methods are common; however, little has been discussed about the limitations associated with these analyses, though it has been highlighted[15, 18]. The use of the DASH in other populations of similar diagnostic groups and interventions is needed to show the degree of consistency in the estimates of treatment effectiveness.