Clarification of the cut-off score for Zung’s self-rating depression scale

Abstract

Background Zung’s Self-rating Depression Scale (SDS) is an established norm-referenced screening measure used to identify the presence of depressive disorders in adults. Despite widespread usage, issues exist concerning the recommended cut-off score for a positive diagnosis. First, confusion arising from the conversion of raw scores to index scores had resulted in a considerably higher cut-off score than that recommended being used by many researchers. Second, research in China [Chin J Nervous Mental Dis. 12:267-268; 2009] and Australia [BMC Psychiatry. 17:329; 2017] had suggested that the current recommended cut-off is lower than ideal, at least in those countries. Method To explore these matters further, sensitivity and specificity figures for alternative cut-off points were examined in positive clinical and negative community samples respectively. The positive clinical sample ( n = 57) consisted of adults receiving treatment from a medical professional for some kind of depressive disorder, whose diagnosis was positively confirmed using the Patient Health Questionnaire (PHQ). The negative community sample ( n = 172) was derived from a representative sample of adults whose absence of any depressive disorder was similarly confirmed by the PHQ. Results Mathematical models, including Youden’s Index and the Receiver Operating Characteristics Curve, suggest that the recommended cut-off (a raw score of 40) is indeed too low. More detailed comparisons, including consideration of the likely numbers of false positives and negatives given prevalence rates, confirm that, ironically, the incorrect SDS cut-off score mistakenly applied by many researchers (a raw score of 50) would appear to provide far greater accuracy. Conclusions Research in China [Chin J Nervous Mental Dis. 12:267-268; 2009] has resulted in an elevated SDS cut-off score of 42 being used in many Chinese studies. Research by Dunstan and Scott [BMC Psychiatry. 17:329; 2017] in an Australian context, suggested that a greater increase, to a raw score of 44 might be required. Based on this study, an even larger adjustment is required. Specifically, we recommend the use of an SDS raw score of 50 as the cut-off point for clinical significance.

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“…Added the total score of these 20 questions and multiply by 1.25 to get the standard score, which indicates depressive symptoms while greater than 53 according to the Chinese version of SDS. 23…”

Norms for Zung’s Self-rating Anxiety Scale

“…Additionally, there is the question as to the number of false positives and false negatives that are likely to have occurred in the PHQ diagnoses. Using the sensitivity (63%) and specificity (97%) figures reported by Spitzer et al [20], it is possible to estimate the approximate number of false positives/negatives that are likely to have occurred in each subsample ( Table 2).…”

“…Examining these results reveals that the case for increasing the recommended clinical cut-off score is far less evident for the SAS than the SDS; see [20] for comparative figures]. While the current cut-off is forecast to produce a sizeable number of false negatives, this has to be balanced against the risk of a number of clinical cases going undiagnosed if the cut-off is raised.…”

“…ROC curves also provide a measure of a test's overall discriminatory ability: the greater the area under the curve, the stronger the test [24]. While the use of ROC curves has its origins in medical disease diagnosis, ROC curves have been successfully applied to explore optimal cut-off scores for psychological screeners (e.g., [20,25,26]).…”

“…Recent research has suggested that the 1980-recommended cut-off point (a raw score of 36 or an index score of 45 [18];) is lower than ideal, and that the original 1971 cut-off (a raw score of 40; an index score of 50 [14];) produces better sensitivity and specificity figures [16]. Similar problems; that is, confusion between index and raw scores and suggestions that the currently recommended cut-off score may be too low, have also been identified for the SDS [16,17,20]. This current study explores this issue for the SAS.…”