We have described the development and preliminary validation of the DASS, a scale to measure satisfaction and quality of life with anticoagulation. Individual DASS items showed sufficient variation, and the large majority of items clearly grouped into scales reflecting positive and negative impacts of anticoagulation. This latter scale can, if desired, be further sub-divided into sub-scales reflecting limitations imposed by anticoagulation versus the hassles and burdens of anticoagulation management. The internal consistency of the overall scale is good (Cronbach's alpha 0.88), with the sub-scales falling into a similar range (alpha 0.78 to 0.91). The sub-scales correlate with various measures of health status and satisfaction with medical care. The level of variation from test to test (intra-class correlation 0.80) is higher than the ideal, but acceptable.
Although these initial results appear promising, various limitations should be noted. Validation is a multi-step process, requiring numerous positive findings, across a variety of applications, before a scale can be invested with full confidence. Some natural follow-up studies would include, among others, administration across a broader cross-section of patients. The DASS does not yet have norms to quantify, for example, clinically significant differences in quality of life, although generic methods might be used as at least a first approximation .
Another limitation pertains to the definition of the sub-scales. Although the overall pattern of the data was consistent with the ideal of simple structure, some items did load onto their respective sub-scales less strongly than others. For some items, this might reflect a moderate ceiling effect. In other cases, this might reflect a tendency for different respondents to conceptualize these items differently. The willingness to accept differing interpretations of the various items was consistent with the philosophy under which the DASS was developed; namely, that in order to be relevant to individual patients and extendable across a wide variety of applications, the scale should address its constructs as generically as possible.
How best to conceptualize quality of life associated with anticoagulation management was an ongoing challenge during the scale development process, a particular difficulty being that our experts (as well as the literature) tended to make somewhat finer distinctions (e.g., between limitations versus hassles and burdens) than were typically made by patients. Our solution was to structure the instrument with into separate sets of questions representing these fine distinctions (e.g., "limitations" comprising one such set), but to retain the option of combining the items in these sets into more general sub-scales. When applying the DASS to other models of anticoagulation management, the questions themselves could be retained (e.g., as they were designed to be applicable across models of care), but some of the text in their stems might be changed. For example, the list of possible daily and occasional tasks would likely differ according to the model of care.
The primary decision for the user that wishes to apply the DASS at the level of the sub-scale is whether to break the "negative impact" scale into two component parts. As discussed above, the structure of the instrument and, indeed, our original expectations regarding the factor analysis, was based on the notion that the negative impact scale would be sub-divided. However, both the original factor analysis and a subsequent confirmatory factor analysis showed somewhat stronger support for the simpler model including positive and negative factors only. The size of the test-retest sample (i.e., 103 subjects) was near the lower limit for a factor analysis, so we do not interpret this confirmatory factor analysis as definitive; nevertheless, its conclusions were quite similar to those of the original factor analysis. Taken as a whole, our interpretation is that a 2-factor solution may be the most natural, but that a user with a specific need to utilize three factors could reasonably do so. The current comparison between the 2- and 3-factor solutions is not definitive and, indeed, it is quite conceivable that choice of sub-scale could differ according to the patient population or the model of anticoagulation management under study.
When considering the sub-scales, the internal consistency, as measured by Cronbach's alpha, approached 0.90 for the negative impacts, hassles and limitations sub-scales, but was closer to 0.80 for the positive impact scale. The lower figure for the positive impact sub-scale might in part be a result of the number of items (i.e., on average, the more items the higher the alpha coefficient), but also because the items address a construct that is broader, and perhaps more subject to individual interpretation, than is the case for the items pertaining to the negative impact of anticoagulation. Authorities disagree on the precise benchmarks that should be applied to psychometric measures such as alpha coefficients (these benchmarks in part depending on the application; for example, with lower correlations being acceptable for scales that are intended to compare groups than for scales use to measure change within individuals). Nevertheless, the internal consistency evidenced by the DASS, both in terms of the rotated factor loadings and the Cronbach's alpha coefficients of its sub-scales, is quite consistent with usual practice for measures that are intended to be used at the level of the group. A similar interpretation applies to the test-retest analysis.
A final challenge in the item development process involved the strong socialization of patients undergoing long-term anticoagulation. Often, patients have been informed that long-term anticoagulation is a medical necessity, without an equally good alternative. (One of the conditions for receiving a heart valve is acceptance of anticoagulation for the remainder of the patient's life. In other circumstances, such as atrial fibrillation, alternatives such as aspirin that are less burdensome yet less effective are available, thus implying that patients that receive anticoagulation have self-selected, at least initially, as perceiving the burdens of this therapy as being less than its benefits.) Any life style modifications (such as eliminating activities likely to result in bleeding and bruising) required by this therapy may have been made long ago, and the effects of these modifications, although initially distressing, may no longer be considered by the patient as reducing quality of life. Nevertheless, it is quite reasonable to speculate that if the patient were managed using a less burdensome model of care, perceived quality of life would improve. Some of the final items in the DASS (e.g., whether the patient would recommend this form of anticoagulation) are an attempt to address this issue, but our talk-through interviews suggested that a non-trivial number of patients found such an exercise in visualization to be conceptually difficult. We know of no ideal solution to this problem, which is by no means limited to the present application.
The clinical relevance of the DASS lies in its ability to summarize satisfaction with anticoagulation and in particular to help identify aspects of anticoagulation that may hinder individual patients from maintaining a PT-INR within therapeutic range. Some of these aspects might be amenable to direct intervention; for example, those patients that find anticoagulation management to be extraordinarily complicated might benefit from either additional anticoagulation-related education, or perhaps from a mode of management that requires less regular testing. Other aspects might not be as directly amenable. For example, those patients who had experienced an outcome such as hospitalization for bleeding or multiple dosage changes during the past year also tended to report higher scores for hassles and limitations; in effect, becoming more sensitized to anticoagulation's potentially negative aspects. However, even in the absence of a more direct intervention, providers might at least maintain increased vigilance for such patients.
In this spirit, providers need not only focus on the negative aspects of anticoagulation, but might also choose to especially reinforce those positive aspects that are considered to be particularly salient by each individual patient. In any event, the ultimate goal is that from identifying the limitations, hassles and burdens, and positive psychological impacts experienced by patients, the cycle of dissatisfaction, leading to poor adherence, leading to poor INR control, leading to poor clinical outcomes, can be broken. The scientific relevance of the DASS is that when interventions to break this cycle are designed, a standard of comparison will be required that is applicable across all the models of care being compared. Our intention is that the DASS can help provide a way forward in developing such a standard. Additional research is needed in order to better understand the relationship between anticoagulation-related quality of life and adherence to treatment regimens, as well as how these regimens can be improved.