Our results provide evidence for good reliability and validity of the Norwegian language version of the DASH in patients with shoulder impingement syndrome. The results are comparable to those reported for the original English version and other language versions.
Reliability
The Cronbach’s alpha coefficient of 0.93 indicated a good internal consistency and is similar to previous reported values. In the original version[
5] and other languages versions[
29‐
31] the reported Cronbach’s alpha was 0.96. A value of 0.93 was also reported for the Norwegian language version in patients with rheumatic diseases[
32]. A Cronbach’s alpha between 0.70 and 0.95 have been proposed as a measure of good internal consistency[
22].
Internal consistency assessed by item-to-total correlations ranged from 0.36 to 0.81.The item-to-total correlations were above the threshold value of 0.3, suggesting that the correlation between each item and the total score of the questionnaire were acceptable. Item-to-total correlations reported for the original English version of the DASH ranged from 0.49 to 0.87[
33]. Values reported for other language versions of the DASH ranged from 0.27-0.88[
30,
34,
35].
The test-retest reliability of the DASH was calculated to 0.89, which is considered to be excellent[
24]. Studies for other languages versions have also shown high test-retest reliability with ICC values varying from 0.82 to 0.96,[
30,
36‐
42]. We retested the patients after approximately one week, which is within the recommended time frame ranging from two days to two weeks[
43]. Due to this short time interval, most of the patients reported their shoulder pain as unchanged at the second visit.
In order to detect any systematic changes, the mean difference between the DASH test and retest was visualized in a limits of agreement plot. The limits of agreement plot may reveal systematic changes between the difference and the average of the DASH or outlying observations. Four out of 59 (6.8%) observations exceeded the limits of agreement. The mean difference between DASH test and retest was 1.1 (95% CI -0.65 to 2.82) and showed no systematic change. There was no apparent tendency for the mean difference to vary systematically with the average score.
The SEM was 4.7 points, SEM
95 was 8.3 and the MDC
95 was 13.1. These results correspond well with the measurement error values reported for the original English version with a SEM of 4.6 points and a MDC
95 of 12.8 points[
40]. The interpretation of SEM
95 is that if a patient has a measured DASH score of for example 50 points at an initial test, the clinician can be 95 percent confident that the patient’s true score lies somewhere between 42 and 58 DASH points. The MDC
95 of 13.1 indicates that the clinician can be 95 percent confident that a change has occured if the measured DASH score at retest has changed more than 13.1 points.
A distinction between MDC and minimally important change (MIC) is useful when interpreting change scores in PROMs[
26]. The MDC is a measure of the statistically important change. The MIC can be defined as the smallest change in score which is perceived as important by patients, clinicians, or relevant others[
26,
44]. Different methods may be used to estimate this threshold value which indicates if a patient is better or worse[
45]. A change above 15 points is found to be above most estimates of MIC for the DASH, and is considered to be the most accurate change score for discriminating between improved and unimproved patients[
5].
Construct validity
Our results of construct validity agree with previous studies[
5]. The expected high positive correlation between DASH and SPADI was confirmed with a correlation coefficient of 0.75. Both the DASH and SPADI intend to measure activity limitations and pain (symptoms). However, there are differences in the content of these questionnaires. The DASH is found to be more wide-ranging than the SPADI and can be linked to 23 categories of the International Classification of Functioning, Disability and Health model (ICF), whereas SPADI is linked to six categories[
46].
We had hypothesized a moderate and negative correlation with the Social Functioning domain of the SF-36, because the DASH is also meant to measure components of the social dimension: family care, occupational and socializing with friends and relatives. A moderate correlation with the Social Functioning of SF-36 has been reported in several other languages versions with correlation coefficients ranging from -0.53 to -0.64[
31,
36,
47‐
49]. The expected moderate negative correlation with the SF subscale of SF-36 was not confirmed. The low negative correlation to the SF (-0.35) may indicate that the Norwegian language version of DASH to a limited degree identifies the social dimension of functional status in this population, as measured by the SF-36.
DASH scores
The DASH questionnaire measures whether the respondent has the capacity to do an activity, regardless of how it is performed. It is scored from 0 (best) to 100 (worst). A mean DASH score of 10 have been reported for the general population of the United States[
33]. A mean score of 13 have been reported for both the general population in Norway[
50] and a working population in Germany[
51]. A Norwegian study of physical function in adult acquired major upper-limb amputees reported a mean DASH score of 22.7[
52]. The mean score of 29.4 (SD ± 13.8) in our study population indicated a more severe level of disability compared with these populations. The level of disability in our study population is comparable to other studies of patients with shoulder impingement syndrome[
53,
54].