Usefulness of the zung self-rating depression scale for schizophrenics

Usefulness of the zung self-rating depression scale for schizophrenics

Yasuhiro Kaneda

Department of Neuropsychiatry, The University of Tokushima School of Medicine, Tokushima, Japan;and Department of Neuropsychiatry, Fujii Hospital, Anan, Japan

Abstract:Objective:This study was performed to provide psychometric data concerning the Zung Self-Rating Depression Scale (SDS) when administered to schizophrenics. Methods:The subjects were 110 schizophrenic inpatients (50 females and 60males) who were diagnosed according to the DSM-IV criteria for schizophrenia. Informed consent was obtained from all subjects. Results:1) Cronbach's alpha for the SDS was .81, and reached the acceptable range for internal consistency. 2) Test-retest reliability coefficient for the total SDS was comparatively high (Spearman's ρ=.87, p<.001, N=19), but those for SDS items revealed variability among items. 3) The mean total SDS score for the 15patients with depression was significantly different from the group of patients without depression. 4) None of the 20 items or total SDS was significantly correlated with extra-pyramidal symptoms. Conclusions:Overall, the findings demonstrate the usefulness of the SDS for schizophrenics. However, due to findings of mixed reliability, caution should be exercised when using the SDS for these patients. J. Med. Invest. 46:75-78, 1999

Keywords:Zung Self-Rating Depression Scale, schizophrenia, reliability, validity

INTRODUCTION
Depressive features are clearly recognized within the context of schizophrenia (1, 2), and clarification of the comorbidity has important clinical implications particularly when choosing the best neuroleptics for treatment. However, in some cases, it is difficult to clearly distinguish between depressive symptoms, negative symptoms and extrapyramidal side-effects (3, 4), resulting in the poor correlation between self-reports and observer reports on depression (5). Therefore, self-rating scales may be more effective than observer rating scales to assess depression in schizophrenic patients.
The Zung Self-Rating Depression Scale (SDS) (6) is a self-reporting scale composed of 20-item scales that was developed to measure depressive symp-toms using 4-point scales. The SDS was originally designed to assess depression in patients with depressive disorders. Psychometric research has provided substantial evidence on the reliability and validity of the SDS with normal subjects and patients with depressive disorders (7). However, data on the psychometric properties of the SDS with schizo-phrenic subjects are in sufficient. This investigation was designed to replicate and extend extant findings regarding the psychometric properties of the SDS among schizophrenics.

METHODS
The subjects were 110 medicated schizophrenic inpatients (50 females and 60 males) that met the DSM (Diagnostic and Statistical Manual of the American Psychiatric Association)-IV (8) diagnostic criteria for schizophrenia. Informed consent was obtained from all subjects. The mean age of the sub-jects was 49.6 years (SD=11.5, range 18-76) (Table1). The mean duration of illness was 19.5years (SD=10.7, range 1-53). The presence or absence of de-pression was determined using the DSM-IV criteria. At the time of assessment, 15subjects showed de-pressive signs and 95 subjects did not.
Test-retest reliability was assessed for 19 patients ;retests were conducted within a mean period of17.0 days (SD=5.6, range 8-25) after the first test.
Extrapyramidal symptoms (EPS) were assessed for 97 patients using the Drug Induced Extra-Pyramidal Symptoms Scale (DIEPSS) (9, 10), which was re-cently established in Japan. The DIEPSS is composed of eight individual parameters and one global asses-sment constructed to measure EPS, using 5-point scales. The mean rating of overall severity was1.1(SD=0.9, range0-3) (Table1).
Statistical analyses were performed using non-parametric tests (Mann Whitney's U-test, Spearman rank correlations).

RESULTS
Table2 shows means and standard deviations of the SDS for the whole sample, the subset of patients without depression and the smaller subset of patients with depression.

1. Internal Consistency
Cronbach's alpha (11) was used to determine the internal consistency of the SDS. Cronbach's alpha for the SDS was .81, and reached the acceptable range for internal consistency.

2. Temporal Stability
Test-retest reliability was calculated for a subset of19patients without depression. Spearman rank correlations for the total SDS scores and each SDS item score were, in ascending order, total=.87 (p<.001), item-1=.34, item-2=.72 (p<.01), item-3=-.09, item-4=-.09, item-5=.84 (p<.001), item-6=.50 (p<.05), item-7=.50 (p<.05), item-8=.64 (p<.01), item-9=.92(p<.001), item-10=.62 (p<.05), item-11=. 80 (p<.01),item-12=.72 (p<.01), item-13=.68 (p<.01), item-14=.61(p<.05), item-15=-.09, item-16=.34, item-17=.66 (p<.01), item-18=.86 (p<.001), item-19=.62 (p<.05)and item-20=.85 (p<.001) (Table2). The correlation for the total SDS was clear, but the coefficients for the SDS items revealed variability among items.

3. Validity
Using the Mann Whitney's U-test, the mean total SDS score for the 15patients with depression was significantly different from the group of patients without depression, with p being <.001.

4. Relationships between SDS scores and EPS
Spearman rank correlations for the total SDS scores and each SDS item score were, in ascending order, total=.06, item-1=-.01, item-2=-.07, item-3=.09, item-4=.13, item-5=.06, item-6=-.04, item-7=.12, item-8=.04, item-9=-.01, item-10=-.02, item-11=-.11, item-12=-.07, item-13=.17, item-14=.14, item-15=-.03, item-16=.11,item-17=.05, item-18=.06, item-19=.06 and item-20=-.02 (Table2). None of the 20 items or total SDS was significantly correlated with EPS.

DISCUSSION
In an assessment of psychiatric symptoms, self-reports and/or observer rating scales are fre-quently used. In particular, observer rating scales are useful, when the levels of psychiatric symptoms are assessed. However, in a schizophrenic sample, prior research has indicated inconsistent agreement between self-reports and observer rating scales of depression. Craig and van Natta (5) reported that schizophrenics revealed no significant correlation between four self-report scales and an observer rating scale of depressed mood. In contrast, when self-report scales are intercorrelated, patients showed highly significant correlations. These results indi-cated difficulty in assessing depressive symptoms in schizophrenia by observer rating scales to assess depression in schizophrenics. One of the main rea-sons for the difficulty was the possible overlap with negative symptoms and extrapyramidal side-effects. Prosser et al. (3) reported that negative symptoms were significantly correlated with some features of depression in schizophrenics, and Van Putten and May (4) reported that patients who developed akinesia with antipsychotic drugs exhibited a significant increase in depressive ratings. Therefore, consider-ing these results and my findings in clinical practice, self-rating scales may be more effective than observer rating scales to assess depression in schizophrenic patients. Thus, the psychometric potential of the SDS, one of the commonly used self-rating scales, was investigated.
In the present study, the internal consistency of the SDS in schizophrenics was high. The test-retest reliability coefficient for the total SDS in schizo-phrenics was also as high as that in normal subjects (r=.85, N=34) as shown by Fukuda and Kobayashi(7). However, test-retest reliability coefficients for the SDS items revealed variability among items. These reliability coefficients suggest that scores on some of the items changed substantially from test to retest. This result may be explained by the pos-sibility that the test-retest interval in some patients was too long and the mental condition of the patients changed slightly due to their psychotic symptoms. In addition, none of the 20 items or total SDS was significantly correlated with extrapyramidal symp-toms.
Overall, the findings demonstrate the usefulness of the psychometric properties of assessment instru-ments when used with populations other than those for which they were designed. However, due to findings of mixed reliability, caution should be ex-ercised when using the SDS with schizophrenics.
Although the SDS is a valuable tool for the as-sessment of depression, the presence or absence of depression must be carefully determined, not by a scale alone. Moreover, observer rating scales are appropriate in patients who are unable to complete self-rating scales.

CONCLUSION
The SDS is a useful tool to evaluate depression in schizophrenics, although caution should be exercised when it is employed.

ACKNOWLEDGMENTS
The cooperation of our colleagues is appreciated. This study was presented at the 37th annual meet-ing of the Tokushima Society of Psychiatry and Neurology, Tokushima, December, 1998.

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Received for publication December 1, 1998;accepted January 7, 1999.

Address correspondence and reprint requests to Yasuhiro Kaneda, M.D., Ph.D., Department of Neuropsychiatry, The University of Tokushima School of Medicine, Kuramoto-cho, Tokushima770-8503, Japan and Fax:+81-88-632-3214.