Background: The Panic Disorder Severity Scale (PDSS) is promising to be a standard global rating scale for panic disorder. In order for a clinical scale to be useful, we need a guideline for interpreting its scores and their changes, and for defining clinical change points such as response and remission.
Methods: We used individual patient data from two large randomized controlled trials of panic disorder (total n=568). Study participants were administered the PDSS and the Clinical Global Impression (CGI)--Severity and --Improvement. We applied equipercentile linking technique to draw correspondences between PDSS and CGI-Severity, numeric changes in PDSS and CGI-Improvement, and percent changes in PDSS and CGI-Improvement.
Results: The interpretation of the PDSS total score differed according to the presence or absence of agoraphobia. When the patients were not agoraphobic, score ranges 0-1 corresponded with "Normal," 2-5 with "Borderline," 6-9 with "Slightly ill," 10-13 with "Moderately ill," and 14 and above with "Markedly ill." When the patients were agoraphobic, score ranges 3-7 meant "Borderline ill," 8-10 "Slightly ill," 11-15 "Moderately ill," and 16 and above "Markedly ill." The relationship between PDSS change and CGI-Improvement was more linear when measured as percentile change than as numeric changes, and was indistinguishable for those with or without agoraphobia. The decrease by 75-100% was considered "Very much improved," that by 40-74% "Much improved," and that by 10-39% "Minimally improved."
Conclusion: We propose that "remission" of panic disorder be defined by PDSS scores of five or less and its "response" by 40% or greater reduction.