For quite some time, I've used Katherine Shear's "Panic Disorder Severity Scale (PDSS)" as my main way of assessing and monitoring the severity of panic disorder.  I've recently woken up to the fact that there is a specifically designed "Self Report" version of this scale.  It is copyrighted, but Dr Shear has given permission for clinicians to use the scale freely in their practice and for researchers to use it in non-industry settings.  For other uses of the scale, Dr Shear should be contacted.  Click on "Panic Disorder Severity Scale - Self Report (PDSS-SR)"  to download a PDF of this excellent assessment measure 

Abstracts and links for four research studies about the PDSS are listed at the end of this post.  Note particularly the 2002 paper showing the comparability of the interview-administered PDSS and the self-report PDSS-SR, and also the 2009 paper giving guidelines on how to interpret different scale scores.  Scores for the PDSS are interpreted somewhat differently depending on whether there's also agoraphobia present or not.  I've put these scoring details onto a downloadable Word document - "Scoring the PDSS".

For a series of other handouts & questionnaires on panic disorder, see this website's page "Panic, OCD & depersonalization information & assessment".  As an aside, I'm puzzled that the major English "Increasing access to psychological therapies" initiative only recommends the "Mobility Inventory" as a specific measure for assessing and monitoring Panic/Agoraphobia - see, for example, page 20 of their "Outcomes Toolkit 2008/9".  This seems a mistake to me.  People can be tortured by panic attacks without necessarily being troubled by agoraphobia (the prime problem assessed by the "Mobility Inventory").  

Furukawa, T. A., M. Katherine Shear, et al. (2009). "Evidence-based guidelines for interpretation of the Panic Disorder Severity Scale." Depress Anxiety 26(10): 922-9.  [Abstract/Full Text] 
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.

Houck, P., R. , D. Spiegel, A. , et al. (2002). "Reliability of the self-report version of the panic disorder severity scale." Depression and Anxiety 15(4): 183-185.  [Abstract/Full Text]  
The interview-administered Panic Disorder Severity Scale (PDSS) has been demonstrated to be a reliable and valid measurement of panic disorder severity. The purpose of this paper is to report on the reliability and usefulness of the self-report form (PDSS-SR). The PDSS and PDSS-SR were administered to 108 psychiatric outpatients with and without panic disorder. The internal consistency of the instruments was analyzed with Cronbach's alpha. Test-retest reliability was assessed on 25 of these subjects by using intraclass correlation coefficient (ICC). In addition, decrease in panic symptoms was measured after cognitive-behavioral treatment (CBT) treatment on 27 subjects. Cronbach's alpha was .917 for PDSS-SR and .923 for the PDSS. The PDSS-SR had good test-retest reliability (ICC=0.81) and was sensitive to change with treatment, with a mean decrease of 7.3 (S.D.=5.1). The self-report version of the PDSS is a reliable format, which could be useful in clinical and research settings.

Shear, M. K., P. Rucci, et al. (2001). "Reliability and validity of the Panic Disorder Severity Scale: replication and extension." J Psychiatr Res 35(5): 293-6.  [PubMed] 
The Panic Disorder Severity Scale (PDSS) is a recently developed seven-item instrument to rate overall severity of Panic Disorder. The scale has previously shown good psychometric properties in a sample of Panic Disorder patients with no more than mild agoraphobia. The purpose of this paper is to confirm reliability and validity, to provide an estimate of a cut-score discriminating the presence or absence of current DSM-IV Panic Disorder, and to determine the factor structure of the instrument. Procedures: 104 psychiatric outpatients, including 54 with current Panic Disorder, underwent structured diagnostic assessment and the PDSS interview. The PDSS was repeated within 3-17 days. Results: we confirmed reliability and validity of the instrument and found a one-factor solution fit the data. A cut-off score of eight identifies patients with current panic with a sensitivity of 83.3%, and a specificity of 64%. Conclusion: the PDSS is a simple, reliable instrument for use in Panic Disorder studies. A cut-score of eight may be useful as a tool to screen patients in settings such as primary care, for diagnosis-level symptoms.

Shear, M. K., T. A. Brown, et al. (1997). "Multicenter Collaborative Panic Disorder Severity Scale." Am J Psychiatry 154(11): 1571-1575.  [Free Full Text]  
OBJECTIVE: To address the lack of a simple and standardized instrument to assess overall panic disorder severity, the authors developed a scale for the measurement of panic disorder severity. METHOD: Ten independent evaluators used the seven-item Panic Disorder Severity Scale to assess 186 patients with principal DSM-III-R diagnoses of panic disorder (with no or mild agoraphobia) who were participating in the Multicenter Collaborative Treatment Study of Panic Disorder. In addition, 89 of these patients were reevaluated with the same scale after short-term treatment. A subset of 24 patients underwent two independent assessments to establish interrater reliability. Internal consistency, convergent and discriminant validity, and sensitivity to change were also determined. RESULTS: The Panic Disorder Severity Scale was associated with excellent interrater reliability, moderate internal consistency, and favorable levels of validity and sensitivity to change. Individual items showed good convergent and discriminant validity. Analysis suggested a two-factor model fit the data best. CONCLUSIONS: The Panic Disorder Severity Scale is a simple, efficient way for clinicians to rate severity in patients with established diagnoses of panic disorder. However, further research with more diverse groups of panic disorder patients and with a broader range of convergent and discriminant validity measures is needed.