Minimal Clinically Important Differences (MCID) in Assessing Outcomes of Post-Traumatic Stress Disorder.

This study sought to determine the minimal clinically important difference (MCID) for two frequently used measures of symptom severity in Post-Traumatic Stress Disorder: the Clinician Administered PTSD Scale (CAPS) and the PTSD Symptom Checklist (PCL). Data from a randomized clinical trial of antipsychotic medication in military-related treatment-resistant PTSD (N= 267) included assessments 4 times over 26 weeks. Methods for estimating the MCID were based on both the anchor-based approach, using the Clinical Global Impressions (CGI) severity and improvement scales, rated by both clinicians and patients; and the distribution-based approach (based on standardized z-scores). Severity and change scores on the CAPS and PCL were converted to z-scores and compared across CGI levels using analysis of variance. The average difference in CAPS z-scores between each of three CGI levels between "moderate" to "severe" and from "no change" to "much improved" was 0.758 for clinician CGI ratings and 0.525 for patient CGI ratings and were similar for the PCL (0.483 and 0.471) with all differences significant at p<.0001). Clinically meaningful CAPS and PCL severity and change z-scores range between 0.5-0.8 standard deviations. The MCID estimates suggested here provide an empirical basis for determining whether statistically significant changes in CAPS and PCL scores are clinically meaningful.