Minimal clinically important difference in obsessive‐compulsive disorder
Authors: Matti Cervin, Eric A. Storch, Lara Farrell, Erik Andersson, Christian Rück, Daniel A. Geller, David Mataix‐Cols
Journal: World Psychiatry (2025)

Researchers and clinicians treating people with obsessive-compulsive disorder (OCD) may want to know the smallest improvement in symptom severity that can be considered clinically meaningful. The concept of minimal clinically important difference (MCID) is gaining traction in psychiatry and is defined as the smallest change in an outcome that a patient or care provider deems beneficial or worthwhile1, 2. To our knowledge, the MCID has not been calculated for OCD. Any MCID calculation should take into account the initial symptom severity, as the amount of improvement likely depends on the starting point. To be clinically meaningful, the MCID should also be validated against other important outcomes such as self-reported symptoms, general functioning or quality of life. In other words, the concept of MCID will only be useful to clinicians if it captures changes that matter to patients. In this study, we calculated the MCID in a large international sample of youth and adults meeting DSM-IV or DSM-5 diagnostic criteria for OCD and with data available at baseline and post-treatment (N=2,136). Participants were drawn from randomized controlled trials and open or cohort studies conducted in Sweden (72%), the US (20%) and Australia (8%). All individual studies were approved by the relevant ethical review boards, and all participants provided written informed consent or assent (depending on age) prior to participation. A majority of the participants were under 18 years old (57%) and female (58%). We used the clinician-rated Yale-Brown Obsessive-Compulsive Scale (Y-BOCS) to calculate the MCID value3, 4. The Y-BOCS is used internationally in OCD research, and is applicable to children, adolescents and adults. Y-BOCS total scores range from 0 to 40, with higher scores indicating higher severity. A score of 14 or higher indicates clinical severity5. In this study, the mean Y-BOCS score at baseline was 23.63±4.83, indicating moderately severe OCD, while at post-treatment it was 13.14±6.88, indicating subclinical to mild OCD5. As an anchor measure of improvement, we used the Clinical Global Impression -Improvement (CGI-I)6, 7, a clinician-rated measure that considers all available information about a patient to rate the degree of improvement or worsening compared to a previous measurement point (in this case, the baseline assessment). The scoring of the CGI-I ranges from 1 to 7, with 1 indicating that a patient is very much improved and 7 indicating that he/she is very much worse. To calculate the MCID scores, participants were categorized as being at least slightly improved (CGI-I score 3 or lower, N=1,886) or unchanged/worsened (CGI-I score 4 or higher, N=250) at post-treatment. Both the raw and the percentage Y-BOCS change score were used as MCID variables. To identify the optimal MCID values, we conducted receiver operating characteristic (ROC) analyses, and calculated the area under the curve (AUC). The Youden index was used to identify the optimal MCID value, as it maximizes sensitivity and specificity simultaneously and is less sensitive to imbalanced groups. The AUC for the raw Y-BOCS change score was excellent (0.92, 95% CI: 0.90-0.94). The optimal MCID value was a 6-point improvement on the Y-BOCS, yielding a sensitivity of 84%, a specificity of 86%, and an overall accuracy of 84%. The AUC for the percentage Y-BOCS change score was also excellent (0.93, 95% CI: 0.91-0.94), and the optimal MCID was a 25% improvement, yielding a sensitivity of 84%, a specificity of 90%, and an overall accuracy of 85%. Next, we estimated MCID separately for those with mild (N=661), moderate (N=1,027) and severe (N=186) OCD at baseline, according to existing benchmarks5. Those with more severe OCD at baseline needed larger improvements to reach the MCID. This was evident for both the raw Y-BOCS change score (severe = 8 points, moderate = 6 points, mild = 4 points) and the percentage Y-BOCS change score (severe = 29%, moderate = 26%, mild = 18%). Similar MCID values were obtained when analyzing separately the subgroups of children and adults (6 vs. 6 points and 25% vs. 27%, respectively). Smaller MCID values emerged for Swedish vs. US/Australian participants (5 vs. 6 points and 20% vs. 26%, respectively), probably attributable to higher mean baseline severity in the US/Australian compared to Swedish participants (25.88±5.35 vs. 22.73±4.29, p<0.001, Cohen's d=0.69). Smaller MCID values also emerged for females compared to males (4 vs. 6 points and 20% vs. 25%, respectively), which was not explained by sex differences in baseline severity (24.00±4.80 vs. 23.68±4.81, p=0.15, Cohen's d=0.07). Because MCID values differed across baseline severity groups, we used the percentage change of 25% for external validation. Participants reaching MCID showed more improvement than those not reaching MCID in self-reported OCD symptoms: the Cohen's d was 1.15 (MCID) vs. 0.25 (no MCID) among children (Obsessive-Compulsive Inventory-Child Version), and 1.31 vs. 0.72 among adults (Obsessive-Compulsive Inventory-Revised). The same applied to self-reported quality of life: the Cohen's d was 0.73 (MCID) vs. 0.22 (no MCID) among children (Child Health Utility, CHU9D), and 0.35 vs. 0.01 among adults (EuroQol-5D, EQ-5D). Similarly, for clinician-rated functioning, the Cohen's d was 1.25 (MCID) vs. 0.48 (no MCID) among children (Children's Global Assessment Scale), and 0.99 vs. 0.23 among adults (Global Assessment of Functioning). The MCID will be useful to clinicians and researchers interested in knowing the smallest amount of symptom improvement on the Y-BOCS that corresponds to a clinically meaningful change. This measure is versatile, because it can be calculated either using raw Y-BOCS points or percentage Y-BOCS change scores, and is invariant across children and adults. However, the practical utility of the MCID may be somewhat limited by the fact that the value varies according to initial symptom severity and, for unclear reasons, also between sexes. It is important to specify that the MCID is not intended to substitute the more stringent constructs of treatment response and remission8, 9, which should always be the ultimate aim of any treatment for OCD.