Because the EQUIPERCENTILE method supposes a continuous distribution of the CGI scores, the range of scores that correspond with, for example, “Moderately ill” (CGI-Severity score of 4), can be read off from the graphs that correspond with 3.5 and 4.5. Based on the present findings, we suggest the guidelines as depicted in and for the interpretation of the PDSS. provides the interpretation of the PDSS total scores for patients currently with agoraphobia and currently without, separately. provides the interpretation of the PDSS percentile changes.
Suggested guideline for interpretation of severity of the PDSS scores
Suggested guideline for interpretation of change of the PDSS scores
These guidelines can give us a hint as to how to define “response” and “remission” if we are to base their judgment on the global severity of the disorder. “Response” is most often interpreted as “Much or very much improved” in terms of the CGI-Improvement, whereas “remission” is thought to be equivalent to “Borderline ill or normal” on the CGI-Severity [17
]. Using these definitions, in conjunction with the analyses presented in this paper, we propose that PDSS percent reduction of 40% be used to identify “response” and that PDSS scores of 5 or less be considered “remission” of panic disorder (we would not consider patients currently with agoraphobia to be in remission).
These evidence-based interpretations are remarkably in line with existing suggestions in the literature. We had formerly defined PDSS response as 40% or greater reduction from baseline in the first report from the MCCTSPD [18
] because this score represented the optimum cutoff in an ROC analysis to detect responders defined as “Much or very much improved” on CGI-Improvement and “Slightly ill” or better on CGI-Severity. Our re-analysis of the MCCTSPD dataset using the EQUIPERCENTILE method confirmed this finding, and a new analysis from our second study (TOPDLTS) replicated this cut score. Yamamoto et al [7
] proposed the following rules of thumb for interpreting absolute PDSS scores: scores up to 10 correspond with “mild,” those between 11 and 15 with “moderate,” and those at or above 16 with “severe” panic disorder. This interpretative guide, although based on only 24 Japanese patients with panic disorder, is roughly consistent with our results based on over 200 American patients. These findings can also be usefully integrated with those of a previous study by the second author (Shear et al 2001) in which we found a PDSS score of 8 accurately identified individuals with, compared to those without, current panic disorder, because 8 represents the most representative point estimate for the “Slightly ill” range among those currently without agoraphobia.
There are several limitations, however. First, we must keep in mind the conceptual difficulty of defining severity, or its change, of a clinical condition by the numeric sum of a rating scale. The psychometric assumption behind rating scales, that individual item scores can be summed to produce a total that has a linear relationship with the clinical phenomenon it was designed to measure, has been called into question [23
]. How to overcome this apparent shortcoming, however, is yet to be specified [25
]. In this article we have followed the standard psychometric conventions to inform the clinical practices and we believe that our emphasis on interpretability of total scores as well as their changes is in accordance with Feinstein’s call for clinical sensibility of rating scales [26
]. Secondly, the PDSS and CGI ratings were not independent, and the same rater completed both scales based on the same assessment interview. All the raters, however, had been trained to reliability and were under continuous supervision throughout the trials. Moreover, whether the same or different raters rate the scale in question and the CGI may not always affect the anchoring. For example, in a similar attempt to provide an interpretative guideline for the Hamilton Rating Scale for Depression, data from studies where the ratings of the two scales were done by one or two raters were quite convergent [16
]. Lastly, the first study (MCCTSPD) included only individuals with mild to moderate agoraphobia and participants currently taking psychotropic medications were excluded. In the second study, however, exclusion criteria were much less restrictive, and participants could have any degree of agoraphobia and current medications were permitted if the patient was willing to consider discontinuation during the open treatment phase.
On the other hand, strengths of our study include the large sample size, rigorously maintained reliability of the PDSS and the CGI ratings by trained raters, psychometrically sound equipercentile linking between the test and the anchor, and replication of the findings across two independent samples.
In summary, we believe that our findings represent a significant pragmatic contribution to the treatment of panic disorder and enhance the ability to link research and practice. The guideline we propose will assist clinical investigators in translating findings to be interpretable by practitioners and patients, and will also support practitioners in their use of the PDSS in management of panic disorder.