Phase I—Translation and cross-cultural adaptationTranslation

Translating the original DASI questionnaire into the Sinhala language was conducted based on the guidelines introduced by Beaton and colleagues (Beaton et al. 1976). The original English version of DASI was independently forward-translated into Sinhala by two qualified, bilingual, native Sinhala-speaking translators. Translation 1 (T1) and Translation 2 (T2) versions were generated by them. Both translators independently forwarded and translated the questionnaire and made a report about doubts and difficulties faced during the translation process. Then a common Sinhala translated version (T3) was created using the original questionnaire and the first and second translator’s Sinhala versions (T1, T2).

The T3 version was back-translated independently into English, by two qualified native English-speaking translators. They were not informed about the objectives of the study and had no access to the original version. Neither of these translators were from a medical background. The back-translated version 1 and version 2 (BT1 and BT2) from the above-mentioned English-speaking translators were then compared with the original version by the review committee. Back translation is only one type of validity check, highlighting main inconsistencies or conceptual errors in the translation.

Cross-cultural adaptation

This phase was conducted using the Delphi technique which is a widely used and accepted method for gathering data from respondents within their domain of expertise (Boulkedid et al. 2011).

Expert committee review

An expert committee consisted of translators, healthcare professionals, and an expert in research methodology all of whom understood the concepts and goals of DASI. The committee assessed the clarity, relevance, consistency, and significance of the items based on the socio-cultural context of Sri Lanka. The experts were requested to review each item in the questionnaire and to indicate whether the item should be retained in the questionnaire to assess the functional capacity of non-cardiac pre-operative patients in Sri Lanka. If they decided that the item should be retained, then they were asked to assess the cultural appropriateness of the words and examples used in the items on a 1–5 scale. If they were assigned a score less than 3, they were further asked to indicate their suggestion on how the item should be modified to improve the cultural appropriateness. Items 3, 7, 11, and 12 had mean scores below 4. Hence, they were modified (Table 1). For instance, in item 3, it was noted that “blocks” were not a standard unit of measurement of distance in Sri Lanka. In item 7, “vacuuming” and in items 11 and 12 “golf”, “bowling” and “skiing” are not activities commonly indulged in in Sri Lanka. They were replaced with equivalent activities for metabolic expenditure. All items of the pre-final version were evaluated and compared to the original version to achieve semantic, idiomatic, conceptual, and content equivalence. The pre-final version was approved by all the members of the committee, stating that all items were clear and would be easily understood by the equivalent of a 12-year-old (Beaton et al. 1976).

Table 1 Modified items of DASI-S versionPre-test

The pre-final version was administered to 10 pre-operative participants to evaluate possible deviations and errors committed during translation. Individuals were tasked with reading and responding to each item to assess whether a clear understanding of the questionnaire alone enabled the respondent to answer accurately, or if other related factors played a role. All participants of the pre-test stated that the items were clear; easy to answer; knew all the activities listed and they had no doubts during application.

Sample size

Sample size was calculated considering the recommendations for the ratio of respondents to items varied, ranging from 5:1 (e.g., 50 respondents for a 10-item questionnaire) to 10:1 (Tsang et al. 2017). Since DASI has 12 items 60 to 120 participants was the range. Eighty-one participants were recruited for the study.

Phase II—validation design, sample, and setting

A cross-sectional study was conducted to assess the reliability and validity of the DASI-S in a group of individuals who were awaiting non-cardiac surgeries. This study received approval from the Ethics Review Committee, Faculty of Medicine, University of Colombo, under reference number EC-22-050. Data was collected from May 2023 to October 2023 by two trained research assistants. The study was conducted at the University Surgical wards, National Hospital of Sri Lanka (NHSL), and Colombo North Teaching Hospital (CNTH) Sri Lanka. A sample of 81 patients was recruited after obtaining the written informed consent to participate in the study.

Measurements

In this study, two instruments were used: DASI-S and the physical functioning sub-scale of 36-item short-form health survey (SF-36) (Vibulchai et al. 2014).

DASI-S

DASI-S evaluated the functional capacity in the areas of personal care, ambulation, household tasks, sexual function, and recreation (Hlatky et al. 1989). Each participant within the patient cohort was asked about their ability to conduct various activities. The response modality employed was binary, denoted as “yes” or “no”. In instances where a participant answered “Yes,” the activity received a score predicated on the established metabolic expenditure associated with each activity (Hlatky et al. 1989). The assigned scores for the activities fell within the range of 1.75 to 8.0. Conversely, if a marked “no”, the score for that activity was zero. The cumulative score, indicative of functional capacity, exhibited a potential range from 0 to 58.2, wherein a score of 0 denoted the poorest functional capacity, and a score of 58.2 denoted the highest functional capacity. Higher cumulative scores were indicative of higher functional capacities (Hlatky et al. 1989).

This cumulative score was used to estimate oxygen consumption (VO2) (ml×kg−1×min−1), which is the rate (V) of oxygen (O2) the body is able to use during exercise/activity. This is an objective measure of cardiorespiratory fitness and functional capacity and can be predicted using the following equation (Riedel et al. 2021) as per the findings of the original DASI questionnaire validation which is expected to hold true in any population.

$$VO2 = 0.43 \times DASI + 9.6$$

SF-36

The Sinhala-translated version of the physical functioning sub-scale of SF-36, which is a 10-item questionnaire in version 2.0 was also administered to the same patients. The SF-36 was translated and cross-culturally adapted into the Sinhala language (Gunawardena et al. 2003). It measures the perceived limitations of an individual, related to physical activities due to health issues (Brazier et al. 1992). Respondents indicate if their health restricts physical activity, basic mobility, and daily living tasks, rating their current limitations on a 3-point Likert scale (yes, limited a lot; yes, limited a little; and no, not limited at all). To score and interpret the results, weighted responses to all questions are totalled to create raw scores, which are then transformed to a 0–100 scale (Ware and Sherbourne 1992, Ware 1976). Higher scores interpret more favourable health status.

Statistical analysis

Statistical analyses were conducted using the Statistical Package for the Social Sciences version 23. Descriptive statistics are presented as percentages, mean plus or minus (±) standard deviation (SD), while ceiling and floor effects were determined by assessing the percentage of participants attaining the maximum and minimum achievable scores. Cronbach’s alpha was employed to evaluate internal consistency.

Construct validity of the instrument was appraised through the convergent validity between the results of DASI-S and the physical sub-scale of SF-36 using Spearman’s rank correlation coefficient and by conducting a factor analysis. Factor structure was explored through principal components analysis with varimax rotation with Kaiser normalization. The combination of Varimax rotation with Kaiser Normalization is often used in exploratory factor analysis to produce a set of factors that are uncorrelated and easier to interpret, while also accounting for differences in variance among factors. This method is widely used due to its simplicity and effectiveness in revealing the underlying structure of a set of variables. The adequacy of the correlation matrix was verified by the Kaiser-Meyer-Olkin criteria, which should be greater than 0.60 and Bartlett's test considering a significance level of 0.05. Eigenvalues greater than or equal to one were considered to extract the relevant factors. Following the rotation matrix, items with a factor loading greater than or equal to 0.4 were added to the factor. To assess known-group validity, the independent sample Mann-Whitney U test was used to compare DASI-S total scores by age group (age < 50 years, age ≥ 50 years). All statistical tests were performed at the two-tailed 5% level of significance and Spearman’s correlation coefficient ± 0.3 to ± 0.5 interpreted as fairly correlated (Akoglu 2018).