Psychometric properties of the Barthel Index for evaluating physical function among Chinese oldest‐old

The Barthel Index (BI) is among the most widely used instruments for evaluating physical function, yet its applicability has not been well studied in the oldest‐old population.


Introduction
The ability to perform activities of daily living (ADL) is an essential component of healthy aging. 1 With the global population ageing, the oldest-old have been the fastest-growing segment and more susceptible to ADL decline and functional dependence. 2 According to China's General Program for Sustainable Development, China was projected to becoming a super-aged society by 2033 and having the largest population of the oldest-olds across the globe. 3 Conveniently and accurately evaluating ADL impairments for the vulnerable oldest-old using appropriate tools requires considerable attention. The Barthel Index (BI), originally developed for stroke rehabilitation, has been widely used for evaluating ADL in healthcare and community settings. 4,5 The BI can be used for a variety of clinical purposes, such as determining the degree of disability, evaluating treatment effects and identifying home care needs. 6,7 In clinical practice, subjects with a total BI score of less than 90 or 95 are considered to be function dependent. Although substantial literature describing the BI's applicability in general population is available, questions remain regarding certain properties among oldest-old population, especially in those from low-and middle-income countries (LMICs).
The BI measures a person's physical function through 10 basic daily tasks (grooming, bathing, feeding, toileting, stair climbing, dressing, bowel control, bladder control, transferring and ambulation) regarding the ability of performing ADL. Identifying the latent categories of functional dependence according to the BI's factor structure is beneficial for targeting long-term care and clinical intervention. However, whether the BI is a single measurement or a multiple-dimensional instrument among oldest-old is still unclear. A number of validity researches demonstrated the unidimensionality of the 10-item BI, 8,9 whereas a few studies from Asian countries including China obtained a two-factor structure. 10,11 Notably, specific self-report ADL tasks (bathing, toileting and stair climbing) are closely related to the living environment and facility, which have been confirmed to be associated with functional disability among older adults in the United Kingdom and China. 12,13 The interaction between living environments and specific ADL task reinforces the need for in-depth analyses of item quality in function assessment tools. Item response theory (IRT) analyses, which have many advantages over the classical test theory (CTT), are applicable for evaluating individual item's contribution to a scale through parameter estimation. 14 In the last two decades, this psychometric methodology has been increasingly applied in comprehensive geriatric assessment. 15,16 The BI has been validated by previous studies among hospitalized and community population. Shahram et al. confirmed the BI to be a reliable and valid questionnaire stroke clinical trials. 17 In another study, the BI showed acceptable fit of a unidimensional structure among geriatric rehabilitation patients. 18 However, the majority of previous validity studies of the BI generally focused on middle-aged or older populations from Western countries, 18,19 with only one study focused on the oldest-old yet with small sample size. 20 As many cultural-related diversity exist regarding residential environments and culture gaps of older people between LMICs and Western world, whether previous results derived from developed countries could apply well to oldest-old adults in LMICs such as China remains unclear. To address these gaps, we comprehensively evaluated the reliability, validity and factorial invariance of the BI in a large sample of the oldest-old and centenarian Chinese population. We also employed the IRT framework to evaluate the quality of each item.

Study populations
The data used in this study was collected from two-stage investigations in the China Hainan Centenarian Cohort Study (CHCCS) from June 2014 to December 2017. The goal of CHCCS is to assess the physical function, mental health, and social status of longevous adults and establish ageing indicators for healthy longevity. Details of sampling strategies, inclusion criteria, and age verification were described in our previously published profile. 21 In total, 1794 old persons aged 80 or above by household register of the Department of Civil Affairs in Hainan were invited to participate, and 794 oldestold participants (aged 80-99 years) and 956 centenarians were involved in the final study. For the centenarian participants, strict three-step age verification methods (Supplementary Figure 1) were adopted to ensure the age authenticity, and 58 subjects who failed to meet the validation were excluded. First round: housed register, identification card, census-derived list; Second round: claimed birthday and Chinese zodiac sign, claimed age when they experienced the specific social upheaval; Third round: milestone assessments such as marriage date, date of first born, subsequent birthdates of children, mother's death. In the current study, 124 subjects who were unable to cooperate due to dementia or paralysis were excluded before the survey, and participants who failed to complete any question in the BI were also excluded. The flow chart of participant recruitment in this study was described in Figure 1.
The ethics committee of the Hainan Hospital of the Chinese People's Liberation Army General Hospital approved the study protocol (serial no. 301hn11201601). All subjects (or guardians) provided written informed consent before participating in the survey. This study followed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) reporting guidelines.

Procedures
For the survey data used in this study, participants were interviewed at their residences or nearby health service centres by a well-trained interdisciplinary medical group. 21 The face-to-face interview was performed by native nurses who were trained in interviewing older adults and able to speak the local dialect. For the ADL measurement, investigators checked the participants' self-report function performance according to their performance during the clinical examinations. There were 3-21 missing values for the 10 items in the BI. Considering the subjects' age and poor health condi- Psychometric properties of the Barthel Index tions, missing data of individual BI questions were coded as incapacity with a score of '0' (dependence). Participants' response and the corresponding missing values are described in Supplementary Table 1. Demographic characteristics including age, gender, ethnicity, education, residence, marital and birth status were collected. To test the criterion-related validity of the BI, related health outcomes (depressive symptoms, subjective well-being and self-report health status [SRH]) were also obtained. Subjects with good literacy and cognitive function completed the survey independently, whereas subjects who were illiterate answered questions with the cooperation of the investigator and their legal representative.

Measurements
We used a Chinese BI version firstly translated by Leung and colleagues in 2007 using standard translation methods 10 to assess the ADL of the participants. The BI consists of 10 items: grooming, bathing, feeding, toileting, stair climbing, dressing, bowel control, bladder control, ambulation and chair-bed transferring. Due to the generally low educational level and potential sensory impairment of oldest-old adults living in Hainan, research team further simplified the linguistic expressions of the BI's questions and responses and tested the feasible and valid in a pilot survey in 2014. 13 The scoring of BI was provided in Supplementary Table 2. In brief, participants' physical performance was assessed on a 2-4 Likert classified response depending on the amount of assistance required for accomplishing the tasks in daily living. Items were scored on a scale of '0', indicating dependence, to '5, 10 or 15', indicating 'needs help' to 'independent'. A higher score indicates higher levels of function, and the maximum summed score of 100 indicates complete independence. When the total score is ≥95, the subject is considered to have good functional ability or independent; participants with a total score of less than 95 were considered dependent.
Depressive symptoms were measured using the Geriatric Depression Scale (GDS-15), which consists of 15 dichotomous items that assess the presence of depressive symptoms in the last week. A higher score indicates more depressive symptoms (a possible range of 0-15; the observation range is 0-15), and as previously described, Chinese oldest-old with a score of 7 points or higher were considered to have a depressive symptom. 22 The Satisfaction with Life Scale (SWLS) consisting of five 7-point Likert items was used to assess participants' subjective well-being aspect (a possible range of 0-35; the observation range is 0-35). SRH was measured using a Visual Analogue Scale (VAS). The VAS score records an individual's self-rated health on a 20-cm, vertical visual analogue scale ranging from 0 to 100, with notes at both ends labelling 'the worst health you can imagine' (at 0) and 'the best health you can imagine' (at 100).

Statistical analysis
Continuous variables with a normal distribution were described as the mean ± standard deviation (m ± SD), continuous variables with a skewed distribution were presented as median (interquartile range), and categorical variables were described as numbers with percentages [n (%)]. As for the skewed distribution of the summed BI score, the Wilcoxon or Kruskal-Wallis rank sum test was used to compare the differences of the BI scores across different groups. Spearman correlation coefficient was used to analyse the correlation between the BI score and other measured outcomes. Internal consistency of the BI was evaluated using Cronbach's α coefficient and item-total correlation. An α coefficient > 0.7 indicates good internal reliability. An exploratory factor analysis (EFA) using the principal component method was performed to explore the factor structure. The eigenvalue criterion (>1) of each factor was used to identify the number of meaningful factors. Confirmatory factor analyses (CFA) with robust weighted least squares estimation were conducted in Mplus (Version 7.4) in cases of violation of the multivariate normality assumption. The χ 2 /df, root mean square error of approximation (RMSEA), comparative fit index (CFI) and normed fit index (NFI) were used to evaluate fitness. The model is considered to have a good fit with a χ 2 /df of 5 or less, an RMSEA of 0.1 and a CFI and NFI greater than 0.90. 23 Factorial invariance of the BI across age, gender, education, ethnicity and residence was tested by a multi-group confirmatory factor analysis (MGCFA) approach, which consisted of a series of nested confirmatory steps. Configural invariance (free parameters), metric invariance (constraints of equivalent factor loadings), scalar invariance (further constraints of the intercepts) and strict invariance (further constraints of residual variances) models were tested across subgroups. A non-significant △ χ 2 (P > 0.05), a △ CFI value < 0.01 and a △ RMSEA value < 0.015 were used to compare the fit of nested models. 24 Since subjects in the current study would not rely guessing to answer questions about physical function, we estimated the discrimination (a) and difficulty (b) parameters of 10 items in the BI using the IRTPRO Version 4.2 software. As the previous research did, a two-parameter logistic (2PL) model was used for two binary items ('grooming' and 'bathing'), and a graded response model was used for the other eight multigraded items. 20,25 In clinical settings, the discrimination between [À2.80, +2.80] is suitable, and the difficulty is acceptable at [À3, +3]. 25 Primarily, we tested the assumptions of an item response analysis. The fit of each item under the particular models was tested using the S-χ 2 statistic suggested by Orlando and Thissen. 26 A non-significant S-χ 2 statistic (P > 0.05) indicates fitness to the model. The unidimensionality assumption was tested according to results from factor analyses. The local independence was confirmed using the χ 2 LD statistic recommended by Orlando. 26 A χ 2 LD statistic < 10 between any two items indicated an acceptable level of local independence, which means that subjects' item responses are independent after controlling for the underlying trait. 27 The item characteristic curve (ICC) was used to establish the relationship between subjects' potential trait and their responses, and the item information curve (IIC) was used to evaluate the measurement precision through the test information function (TIF). The measurement precision of a scale is sufficient when the total test information is above 25. 28

Reliability, validity and factor structure of the BI
The summed BI score was significantly correlated with GDS-15 (r = À0.36, P < 0.001), SWLS (r = 0.23, P < 0.001) and VAS (r = 0.22, P < 0.001). As Table 2 showed, the overall α coefficient of the BI was 0.903 and increased only when 'stair climbing' was deleted. The item-total correlation coefficients ranged from 0.508-0.789 (Ps<0.001). Two factors explaining 73.64% of the total variance were retained in the EFA, and all item loadings on the corresponding factor were greater than 0.6. Factor 1 consisting of eight items (grooming, bathing, toileting, dressing, feeding, stair climbing, transfer and ambulation) was defined as 'somatic function'; Factor 2 consisting of two items (bowel control and bladder control) was defined as 'physiological self-care'. Considering previous studies obtained a unidimensional structure of the BI and the item 'stair climbing' impaired the overall internal consistency in the current study, we involved four competition models to explore the optimal structure of the BI in CFAs. Model A: two factors with 10 items; Model B: delete 'stair climbing' from Model A; Model C: one factor with 10 items; Model D: delete 'stair climbing' from Model C. As described in Table 3, all the four CFA models showed acceptable RMSEA (<0.1), CFI(>0.9) and NFI (>0.9), whereas the Model A had a smaller χ 2 /df (5.538) and RMSEA (0.051). Next, we tested the factorial invariance of the two-factor BI model (Model A) across age, gender, education, residence, and ethnicity using MGCFA ( Table 4). The configural invariance model (free parameters) was used as a basic model, and three restrictive models (restrict loading, intercept and residual sequentially) were tested in a stepwise manner. The invariance of metric, scalar, and residual restricted models was confirmed (△CFI < 0.01, △NFI < 0.01, △RMSE<0.015) across residence and ethnicity subgroups. For gender and education subgroups, the invariance of metric and scalar restricted models was confirmed, but the residual restricted model was not (△CFI > 0.01, △NFI > 0.01). For age subgroups, the metric restricted model met the invariance criteria (△CFI = 0.010, △NFI = 0.009, △RMSE = 0.011).

Psychometric properties of the Barthel Index
Item characteristics of the BI As described in Supplementary Table 3, no statistically significant S-χ 2 was found for the 10 items (ranged from 10.05 to 41.41, Ps > 0.05), which indicated that the parametric models were fit to the data. In the EFA, the ratio of the first eigenvalue (6.014) to the second eigenvalue (1.351) was greater than 3 ( Table 2), and the one-factor Model C (CFI = 0.988, RMSEA = 0.057) showed acceptable fitness. Thus, the unidimensional hypothesis was supported by Hambleton's criteria. 29 As Supplementary Table 4 described, the χ 2 LD statistic between 'bowel control' and 'bladder control' was 63.7, which was the highest, and the χ 2 LD statistic between any other two items (0.1-8.4) were all less than the criterion of 10. The ICC and IIC of 10 items in the BI were summarized in Figure 2, and estimated parameter values were shown in Table 5. The discrimination parameter of 10 items ranged from 1.62 ('stair climbing') to 5.87 ('bathing'), and the difficulty parameter ranged from À2.65 to 1.11. The test information provided by 'stair climbing' (1.73), 'bowel control' (2.19) and 'bladder control' (2.56) was relatively low compared with others. As Figure 3 showed, the to-tal test information of the BI was 43.85, and the corresponding latent trait level (θ) located at À0.94. When 'stair climbing', 'bowel control' and 'bladder control' were deleted from the BI, there was no obvious loss of the total information function.

Discussion
This is the first study to evaluate the psychometric properties of the BI among oldest-old and centenarian populations with a considerable sample size. We observed significant variances of the participants' ADL score across age, gender, education and health status subgroups, which confirmed the known-group validity of the BI. Significant correlations between the BI score with measurements of GDS-15, SWLS, and VAS supported the BI's criterion-related validity with was reported in most previous validity studies. 30,31 The BI had a satisfactory internal consistency coefficient of 0.903, which was generally consistent with that reported in previous studies ranging from 0.69 32 to 0.96. 33 The dimensionality of The mean item-total correlation of the Barthel Index. the BI is not consistent across all the studies. EFA and CFA showed that the two-factor structure was an optimal solution in Chinese oldest-old population. Studies from Iran, 11 Sri Lanka 34 and China 10 obtained the same two-dimensional structure in older adults. However, a number of studies confirmed the unidimensionality of the BI in neurology rehabilitation patients. 8,35 Conceptually, somatic disability and incontinence are two latent categories of functional dependence in this population. Factor 1 ('somatic function') mainly reflected the performance of limb muscles, whereas factor 2 ('physiological self-care') represented the performance of the sphincter. The composition of items contained in the above two factors was consistent with Leung et al.'s study among stroke inpatients. 10 But grooming and two incontinence items were extracted into the same factor in Sakar et al.'s study among outpatients. 11 According to the World Health Organization's International Classification of Functioning, Disability and Health, incontinence was classified as impairment of physiological function rather than restricted mobility. 36 Among the 856 functional dependent oldest-old in our study, 163 had incontinence and they needed more targeted long-term care. Our results highlighted the definition and scoring approach of the measurement when using the BI among Chinese community oldest-old. We used the MGCFA to test the BI's factorial invariance that were not well reported by previous psychometric studies. The two-factor structure showed acceptable invariance across age, gender, education, residence, and ethnicity subgroups, which supported the comparability and robust property of the BI. For instance, despite concerns that demographic differences exist between the oldest-old and centenarians, the age invariance indicated that subjects across the two subgroups responded to the scale with the same underlying framework. In IRT analyses, the highest χ 2 LD statistic (63.7) between 'bowel control' and 'bladder control' indicated that they seriously destroyed the BI's local independence. Local independence of a valid scale means that the subject's latent trait is the only factor affecting their response. 26 In terms of physiological function of the human body, 'bowel control' and 'bladder control' were not only related to the ability to take care of oneself but also to factors including spinal injury or neurological function. In a Rasch analysis study among hospitalized older patients, these two continence items also appeared to Psychometric properties of the Barthel Index measure a different concept of the other items. 37 Hartingsveld et al. 38 also found that incontinence items impeded the BI's one-dimensional property. Although 'bowel control' and 'bladder control' provided less information than other somatic items ( Table 5), these two incontinence items are necessary to identify potential categories of disability when using the BI. All the 10 BI items showed sufficient discrimination parameters (>1). Another IRT study conducted by Liu and colleagues found that most items in the BI showed similar good discrimination, but two items ('stair climbing' and 'feeding') had very low discrimination parameters among older persons from long-term care facilities. 20 The common low difficulty parameter found in the current study (À2.65 to 1.11) was derived from the ceiling effect of the BI, which has been frequently reported in previous validation studies. 39,40 The BI had sufficient measurement precision with a total test information of 43.85. In Liu's study, 20 the BI's information function was as sufficient as ours (TIF > 40), and the corresponding location was also relatively low (θ < 0). Notably, the TIF of the 10 individual items ranged from 1.37 to 10.22, which meant that the contribution of individual items to the scale was quite different. Particularly, the TFI of 'stair climbing' was the lowest, and this item impaired the overall internal consistency. The majority of partic-  ipants in the current study lived in bungalows and did not need to climb stairs. Therefore, 'stair climbing' seemed to be a redundant task in population from current study. Although this findings might not be completely applicable to other older groups, we argued that living conditions were to precise measurement of ADL for a specific population. Similar viewpoints were reported in Tomoko et al.'s study, 41 which focused on individuals that lived in long-term healthcare facilities that had either an elevator or no stairs. In this study, we provided psychometric evidence and methodology suggestion for the development and modification of function assessment item banks. Comparability of ADL measurement should be linked to the diversity of living facilities for older people in different areas. For conveniently measurement purpose, researchers have suggested that the standard BI versions should be modified to accommodate different populations with varied living settings. 42 A recent systematic review found that six simplified versions of BI containing three to eight items were also used well in patients with stroke. 37 One strength of this study is the unique study population with a considerable sample size. Another strength is the combination of psychometrical methodologies of CTT and IRT. This study has some limitations. Firstly, the sample used in this study originated from one province in China, and generalizability to older people with varied background needs to be further verified. In any case, we have provided commonly practical methodological suggestions for assessing and optimizing ADL assessment tools. Secondly, due to the subjects' advanced age and cognitive impairment, information bias was inevitable. However, we have adopted strict quality assurance approaches to ensure the authenticity of subjects' ages and ADL measurements during the investigation. Thirdly, clinical factors such as chronic diseases, geriatric syndromes and polypharmacy were not included, which might influence the psychometric evaluation of the BI. Fourthly, we could not analyse the test-retest and inter-rater reliability of the BI due to the lack of repeated measurement data. Future studies are justified to investigate this issue.

Conclusion
The BI has appropriate reliability, validity and measurement precision for community-based Chinese oldest-old and centenarians. Somatic disability and incontinence are two latent categories of functional dependence in this population. Living environment needs to be taken into consideration for ADL instrument development and modification.

Online supplementary material
Additional supporting information may be found online in the Supporting Information section at the end of the article. Figure S1. Three-steps age validation process of centenarians in CHCCS. Table S1. 1750 participants' response ratio of 10 items in the Barthel Index Table S2. Rating scale of activities of daily living by the Barthel Index Table S3. S-χ 2 statistics diagnostic statistics of 10 items in the Barthel Index Table S4. The χ 2 LD statistic matrix of 10 items in the Barthel Index