This study demonstrates that the Thai version of the CDSS is a reliable and valid measure for the evaluation of depression in Thai patients with schizophrenia. The internal consistency of the Thai version of the CDSS, which indicates the agreement among the individual components of the measure, was very good (Cronbach’s alpha = 0.869). This result supports the high internal consistency of the CDSS, which was also found in other versions, including the Japanese version (Cronbach’s alpha = 0.82), the Greek version (Cronbach’s alpha = 0.87), and the French version (Cronbach’s alpha = 0.82).16
All the items appeared to correlate well with the scale overall, although the item-total correlation was low in early wakening (C7). This finding was similar to that reported in the French version.16
The small item-total correlation in C7 might be due to use of benzodiazepines, which were highly prescribed in this population, as well as the low severity of depressive symptoms and psychotic symptoms found in the study participants.
The overall inter-rater reliability of the Thai version of the CDSS was found to be in substantial agreement (ICC = 0.979) and the ICC for each item was also high. These findings are similar to those for other versions of the CDSS.16
The test-retest was done at intervals of 3 days or more. Although depression in patients with schizophrenia might change over time, the test-retest reliability for the total score of the Thai version of the CDSS in this study was high, with an ICC of 0.861.
The mean total score of Thai version of the CDSS in this study significantly correlates with those of the MADRS, the HDRS-17, and the PANSS-G6. These results are similar to those of Addington et al, who reported significant relationships between the CDSS, HDRS, and Beck Depression Inventory7
Our study also found significant correlations between the Thai version of the CDSS and the PANSS score (positive, negative, general psychopathology, and total score). The relationship between the CDSS and PANSS scores identified in previous studies has been controversial. Lançon et al found significant correlations between the CDSS, the PANSS positive subscale, the PANSS general psychopathology subscale, and PANSS total score,20
while Addington et al reported a significant correlation between the CDSS and the PANSS negative subscale.7
Additionally, some items of the CDSS, including guilty ideas of reference (C4) and early wakening (C7) were found to be significantly correlated with the PANSS positive subscale when the patients were in remission.21
These correlations might represent the contamination of the CDSS by the psychotic symptoms in patients with schizophrenia. Nevertheless, there was no significant correlation between the CDSS and the PANSS positive and negative subscale, and the Scale for the Assessment of Negative Symptoms (SANS) score on admission.21
The DSM-IV-TR criteria for major depressive episode were utilized as a gold standard following the original study of Addington et al, which used DSM-III-R criteria.10
It was found that the area under the ROC curve of the CDSS against the DSM-IV-TR criteria for major depressive episode was higher than that of the other depressive measures. This indicates that the Thai version of the CDSS is more effective than the MADRS, HDRS-17, and PANSS-G6 in evaluating depressive symptoms in patients with schizophrenia. The cut-off point of 6/7 for detecting major depressive episodes provided a high sensitivity (92.31%) and specificity (97.87%) as well as a high positive predictive value (92.31%) and negative predictive value (97.87%).
Some limitations should be taken into account when interpreting the findings of this study. First, the depression and psychotic symptoms of the patients with schizophrenia in this study were moderately low. This problem has been reported in studies looking at other versions of the CDSS and may limit the use of the scale in patients with severe depressive and psychotic symptoms.16
However, it may have an advantage in detecting borderline cases of depression. Second, the particular sample characteristics from a university hospital in Thailand might reduce generalization to other patients with schizophrenia in different settings. Third, some of the patients were being treated with antidepressants and benzodiazepines during the evaluation, which may have interfered with some items in the CDSS. As such, further study of the reliability and validity with a wide range of depressive and psychotic symptoms in different settings, such as in a psychiatric hospital, should be conducted to confirm the results of this study.