Demographic characteristics and somatization scores of FD patients
Herein, 612 patients were diagnosed with FD according to Rome IV criteria. 17.3% (106/612) of patients were identified with anxiety disorders, 13.6% (83/612) of patients had comorbidity of depression, and 7.8% (48/612) of patients were diagnosed with comorbidity of anxiety and depression. Among 471 FD patients without comorbidity of anxiety and depression, 63.5% (299/471) and 36.5% (172/471) of patients were female and male, respectively. The proportions of FD patients’ level of education for primary school, middle school, high school, college or above were 22.7% (107/471), 47.3% (223/471), 14.4% (68/471), and 15.5% (73/471), respectively. The patients’ mean age was 43.4±10.3 years old (range, 18-67 years old). Moreover, 20.6% (97/471) of patients were diagnosed with EPS, 31.0% (146/471) with PDS, and 48.4% (228/471) with both EPS and PDS. The FD patients’ demographic characteristics used for the PHQ-8 and the SSS-8 are shown in Table 1.
Among the scores evaluated by these two scales, a significant difference was found in gender (P<0.05). However, there were no significant differences between different ages and levels of educational (P<0.05).
Items used for developing the PHQ-8 were selected by the four methods
Using discrete degree method, chest pain, shortness of breath, fainting, palpitation, sexual life pain or other discomfort, numbness/tingling, excessive sweat, heavy hands/foot, a bursts of cold/fever, memory loss/forgetfulness, urinary frequency, urinary pain /dysuria, blurred vision, neck and shoulder pain, and muscle pain were removed since the SD of the items was defined < 0.5.
Using correlation coefficient method, fainting (r=0.090), dysmenorrhea or menstrual other discomfort (r=0.243), excessive sweat (r=0.296), and urinary pain /dysuria (r=0.260) were removed.
According to factor analysis method, the value of KMO value for the initial scale was 0.668, and the value of Bartlett’s spherical test was 366.894, P < 0.01, which was appropriate for factor analysis, and indicated that each item had a factor loading of more than 0.4 in its dimension except for headaches and chest pain.
Cronbach’s α coefficient method showed that the α coefficient increased after removing fainting and urinary pain /dysuria.
The screened items of the developed PHQ-8 are presented in Table 2, including back pain, pain in arms, legs, or joints, dizziness, fatigue, dry mouth, feeling tired or having low energy, insomnia or other sleep problems, and throat discomfort.
Reliability analysis of the developed PHQ-8 and the SSS-8
Intrinsic reliability analysis
Cronbach’s α is a measure of the internal consistency or reliability. The Cronbach’s α test is typically utilized to examine the consistency and stability of the questionnaires. Hence, the Cronbach’s α was herein applied to ascertain whether the items were reliable in measuring the same dimension. It is generally believed that when the Cronbach’s alpha coefficient is greater than 0.7, the reliability is satisfactory [48]. The Cronbach α coefficient of the developed PHQ-8 and the SSS-8 was 0.601 and 0.553, respectively. The correlation coefficient between each item and the total score was 0.426~0.652 and 0.359~0.573, respectively.
Criterion validity
Criterion validity unveiled that the correlation coefficient between the total score of the PHQ-8 and the SSS-8 and the PHQ-15 was (r=0.739, P=0.000) and (r=0.835, P=0.000), respectively. As shown in Table 3, the developed PHQ-8 outperformed the SSS-8.
Structural validity analysis for the developed PHQ-8 and the SSS-8
Exploratory factor analysis: The developed PHQ-8 showed to have values of 0.668 and 366.894, (P<0.01) in KMO and Bartlett’s spherical tests; for the SSS-8, the corresponding values were 0.680 and 236.445 (P<0.01), respectively. The exploratory factor analysis was carried out on the scale, and the principal component method was used to maximize the orthogonal rotation through the covariance matrix and the variance, in which the common factor was extracted by using the Kaiser criterion (Eigenvalue > 1). Additionally, 3 common factors of the developed PHQ-8 were extracted, and the cumulative contribution rate was 55.103%. The range of factor loading was 0.482~0.802, which was higher than the minimum standard of structural validity test equal to 0.4 [46,48]; Besides, 3 common factors for the SSS-8 were extracted, and the cumulative contribution rate was 51.666%. The range of factor loading was 0.353~0.881. All coefficients related to factor loading for PHQ-8 and the SSS-8 are summarized in Tables 4 and 5.
Confirmatory factor analysis: Confirmatory factor analysis model often employs values of chi-squared values (c2), root mean square residual (RMR), root mean square error of approximation (RMSEA), goodness-of-fit index (GFI), adjusted GFI (AGFI), comparative fitting index (CFI), Tucker-Lewis index (TLI), normed fit index (NFI), and other indicators were utilized to evaluate the fitting effect of the model. The smaller the value of c2, and GFI, AGFI, CFI, TLI, NFI > 0.9 indicated that the model fitted well, the closer to the “l”, the better; besides, RMR and RMSEA < 0.05 indicated that the model fitted well, the closer to “0” fit, the better. The developed PHQ-8 and the SSS-8 were separately assessed by exploratory factor analysis, and 3 common factors were extracted. Two scales confirmatory factor analysis had a good 3-factor model fit, shown in Table 6.
Correlation analysis of anxiety, depression, somatization, and the DSS for the developed PHQ-8 and the SSS-8
Somatization was evaluated by the developed PHQ-8 and the SSS-8. Correlation analysis showed that anxiety, depression, and somatization were positively correlated with the DSS (Table 7). The correlation coefficient between the DSS and somatization assessed by the SSS-8 was higher than that of the developed PHQ-8.
Linear regression analysis of the effects of anxiety, depression, somatization on the DSS
Somatization, anxiety, and depression were taken as independent variables, and the DSS was taken as a dependent variable, and linear regression analysis was conducted by the backward elimination method. After adjusting for factors, such as gender, age, type of FD, level of education, and employment situation, it was uncovered that depression and somatization were found as factors influencing the DSS (Table 8). The adjusted R2for the PHQ-8 and the SSS-8 were 0.263 and 0.263, respectively (all P<0.001). It also was unveiled that the role of somatization might be more significant than depression. The standardized β for the SSS-8 seemed to be higher than that for the PHQ-8.
Correlation analysis of anxiety, depression, somatization and QoL
Correlation analysis showed that somatization, anxiety and depression were positively correlated with the NDI (Table 9). Compared with the SSS-8, the correlation coefficient between NDI and somatization for the developed PHQ-8 was higher.
Linear regression analysis of the effects of anxiety, depression, somatization on QoL
The NDI for QoL was taken as a dependent variable, and somatization, anxiety and depression were as independent variables. The linear regression analysis was undertaken by backward elimination method. After adjusting for factors, such as gender, age, type of FD, level of education, and employment situation, somatization, anxiety and depression were noted as factors the influencing QoL (Table 10). The adjusted R2 for the PHQ-8 and the SSS-8 were 0.224 and 0.236, respectively (all P<0.001). Somatization appeared to be more important than anxiety and depression. The standardized β for the PHQ-8 was greater than that for the SSS-8.