Sample and study design
Stratified multi-stage random sampling was done in Shaanxi province of China. It is one of the most endemic KBD areas of China (Chinese Kashin–Beck Disease Surveillance Group 2003, 2006). Two counties were randomly selected from 30 most prevalent KBD counties of Shaanxi province, and then six villages were randomly selected from two counties, three villages from each county. The diagnosis and degree of severity of KBD were assessed based on the standard KBD diagnostic criteria: an individual who has resided in a KBD endemic area for at least 6 months showing the following clinical or radiological changes. (1) Radiological changes in the distal end of the bones of the middle and proximal phalanges of the index and ring fingers. (2) Focal or irregular premature closure of the epiphysis. (3) Limited motion and enlargement of peripheral joints, deformities and dwarfism. (4) Involvement of multiple joints by non-inflammatory lesions (Guo 2001). The KBD was divided three degrees according to severity of disease, higher degree means more severity. The criteria of 1st degree are enlarged finger joints, arthritic pain in the knee and ankle joints, and abnormal X-ray signs (metaphysical lesions in phalange). The 2nd degree KBD patients exhibit shortened fingers and clinical symptoms of 1st degree and worse X-ray signs than 1st degree patients. The criteria of 3rd degree are retarded growth or dwarfism and clinical symptoms of 1st and 2nd degree and worse X-ray signs than 2nd degree.
The individuals were diagnosed by a physician with expertise in the diagnosis of KBD. Inclusion criteria were: diagnosis of KBD and greater than 18 years of age. Exclusion criteria were: inability of understanding questionnaires. Questionnaires included the EQ-5D-3L (Chinese version) (EuroQol Group 2013), WHOQOL-BREF (WHOQOL group 1996) and a general profile questionnaire. Because more than 70.0% of individuals with KBD had low educational level of primary or lower (Fang et al. 2012). They were not able to read the questionnaires. Thus, this study was conducted by face to face interview. The interview was performed using Mandarin (Chinese) at local community hospital. The interviewer filled the general profile questionnaire then explained the WHOQOL-BREF and EQ-5D to the respondents and asked them to fill in the WHOQOL-BREF, EQ-5D and VAS.
The interviewers were graduate students of the department of public health of Xi’an Jiaotong University and staff of the department of public health of local community hospital. They were trained by the researchers.
For assessing test–retest reliability (Streiner and Norman 2008), a second interview was performed at the same place by the same interviewers 10–14 days after the first interview. Before performing retest, all individuals were asked if there was any change in their health status since the last assessment. Those who reported any change were excluded from retest.
The Ethics review Committee of Xi’an Jiaotong University School of Medicine approved the study. A thorough explanation of the study was provided to potential subjects. Informed consent was acquired in writing from all the participating individuals.
The general questionnaire included information about the socio-demographic profile, KBD degree, painful joints and the general health item with response options ‘very good’, ‘good’, ‘fair’, ‘poor’, ‘very poor’.
The EQ-5D-3L (EuroQol Group 2013) consists of five dimensions with three-level options (EQ-5D-3L) and EQ VAS (visual analogue scale). The five dimensions include Pain/Discomfort, Self-care, Usual Activities, Mobility and Anxiety/Depression. Scores for the five dimensions, by applying EQ-5D-3L value sets, can be transformed into index scores that range from −0.149 to 1, higher score means better QOL. China EQ-5D-3L value sets were used in this study. It generated using the time trade-off method and published in 2014 (Liu et al. 2014). Before 2014, UK, Japan or USA EQ-5D-3Lvalue sets were most applied as a substitute (Wang et al. 2012; Cao et al. 2012; Jin et al. 2012).
WHOQOL-BREF is a validated instrument for Chinese population (Fang et al. 1999; Power et al. 2006). It contains a total of 26 questions with four domains (psychological health, physical health, social relationships, and environmental) and two general items (G1 and G4) which inquire about overall perception of an individual’s own QOL or health (WHOQOL group 1996). The four domain scores denote an individual’s perception of quality of life in each particular domain. Domain scores range from 0 to 20, and are scaled in a positive direction (i.e. higher scores denote higher quality of life). Domain scores were used to analyze data from this scale.
Validity and Reliability of the EQ-5D-3L were evaluated using established guidelines (Scientific Advisory Committee of the Medical Outcomes Trust 2002).
The reliability of test–retest was measured by calculating the Kappa coefficients (Cohen 1968) for EQ-5D-3L five dimensions between test and retest response and percentages of agreement. Kappa value 0.81–1.0 indicates an almost perfect agreement, 0.61–0.80 substantial agreement, 0.41–0.60 moderate agreement, 0.21–0.40 fair agreement, below 0.20 indicates a slight agreement (Landis and Koch 1977).
For the EQ VAS, the intraclass correlation coefficient (ICC) (Fleiss and Cohen 1973) was computed. An ICC of more than 0.80 indicated excellent reproducibility, and one between 0.61 and 0.80 moderate reproducibility and one between 0.41 and 0.60 fair reproducibility (Shrout 1998).
Convergent and divergent validity
Convergent and divergent validity were examined by the correlations between EQ-5D-3L dimensions, EQ VAS and previously validated WHOQOL-BREF using Spearman’s rank correlation. We expected that comparable dimensions would correlate better than less comparable dimensions. Because four items of the EQ-5D (mobility, usual activity, self-care, and pain/discomfort) are chiefly correlated with the physical domain of the WHOQOL-BREF, the anxiety dimension of EQ-5D is chiefly correlated with the psychological domain of the WHOQOL-BREF, we expected the following: (1) The four items (self-care, usual activity, mobility, and pain/discomfort) of EQ-5D-3L, index scores, and VAS would show moderate to high correlations with the physical health domain of the WHOQOL-BREF as convergent; (2) The anxiety of EQ-5D-3L would show moderate to high correlations with the psychological domain of the WHOQOL-BREF as convergent; and (3) Because five items of the EQ-5D are not directly correlated with the environment and social relationships domain of the WHOQOL-BREF, we expected the 5 items of EQ-5D-3L, index scores, and VAS would show weak or no correlations with this two domains as divergent.
Spearman’s rank correlation coefficient of 0.5 or above indicates high correlation, 0.30–0.49 moderate correlation, and lowers than 0.30 weak correlation (Cohen 1988).
Known groups’ validity
The validity of identified groups’ was evaluated by comparing the EQ-5D-3L index scores and EQ VAS for subgroup with different KBD degree, the general health and number of painful joints using Kruskal–Wallis tests. We expected individuals with more severe disease, poorer general health, and a greater number of painful joints would have lower EQ-5D-3L index scores and EQ VAS scores.
All data analysis was carried out using SPSS16.0. P values <0.05 were considered statistically significant.