When assessing and monitoring progress with OCD sufferers, originally I used the Y-BOCS questionnaire.  Then some years ago I switched to using the OCI (distress scale).  This is fine ... it's the officially recommended OCD questionnaire by IAPT, the English Increasing Access to Psychological Therapies initiative (see pages 39-41 of their freely downloadable Appendices and helpful resources manual).  But a hassle about the 42-item OCI is that it's time consuming, taking about 12 minutes to complete and a fair amount of time to review & score.  I've now shifted to using the 18-item OCI-R (see Veale et al, 2016 below). 

In making this change, I'm reassured by the findings of the 2012 paper "Assessing obsessive-compulsive disorder (OCD): A review of self-report measures" which compared ten different questionnaires.  The authors concluded - "For time efficient measures, the OCI-R is recommended as it has the best psychometric properties of the briefer measures, with numerous external validation studies in English and in other languages".  They estimate that it takes just 3 to 5 minutes to complete the OCI-R.  One can drop down even further to the 3-item OCI-R Main if time is really short.  The very focused OCI-R Main just uses the three questions relevant to the most severely affected OCI-R subscale (washing, checking, ordering, obsessing, hoarding & neutralizing).  If scores for two or more subscales are equally bad, the OCI-R Main score is calculated as the mean of the relevant subscale scores (both pre-, during and post- treatment).  For child and adolescent populations there's the 21-item child version - the OCI-CV.  

Here's a downloadable copy of the 18-item OCI-R (as a Word doc & as a PDF file) and here the 42-item OCI (as a Word doc & as a PDF file).

Each of the OCI questions is answered using a 0 (not at all) to 4 (extremely) distress scale.  IAPT defines 'clinical caseness' on this full 42-item scale as a score of >=40, with 'reliable improvement' involving a change in score of >=32.  Veale et al (see below) recommend that caseness on the OCI-R be defined as >=17 and reliable improvement as a reduction in score of >=13.  Interestingly, in Veale's IAPT outpatient population (not his severe refractory inpatient group), the average length of treatment was 14.8 sessions with a minimum number of sessions of 9 and a maximum of 26.  It was noteworthy that, in this expert specialised service, 65.1% achieved reliable improvement on the OCI-R, 50.8% recovered, and 42.9% achieved both reliable improvement and recovery. 

Veale, D., et al. (2016). "Sensitivity to change in the Obsessive Compulsive Inventory: Comparing the standard and revised versions in two cohorts of different severity"  Journal of Obsessive-Compulsive and Related Disorders 9: 16-23.  The Obsessive Compulsive Inventory (OCI) is often used as a screening instrument for symptoms of Obsessive–Compulsive disorder (OCD) and as an outcome measure for treatment. Three versions of the OCI are available: the original 42-item version, the revised 18-item version (OCI-R) and a shorter version that focuses on the highest subscale (OCI-R Main). Our aim was to determine sensitivity to change and evaluate cut-off scores for caseness in each version of the OCI using the same dataset. Method: We compared the effect size and the number of patients who achieved reliable and clinically significant change after cognitive behavior therapy in two samples of out-patients with OCD. One sample (n=63) had OCD of minor to moderate severity and a second sample (n=73) had severe, treatment refractory OCD. Results: The OCI-R is a valid self-report outcome measure for measuring change and is less burdensome for patients to complete than the OCI. Questions remain about whether the OCI or OCI-R is sufficiently sensitive to change for a service evaluation. We would recommend a slightly higher cut-off score of ≥17 on the OCI-R for the definition of caseness. Discussion: In both samples, the OCI and OCI-R had very similar treatment effect sizes and to a lesser extent in the percentage who achieved reliable improvement and clinically significant change. The OCI-R Main was more sensitive to change than the OCI or OCI-R in both samples. All versions of the OCI were less sensitive to change compared with the Yale-Brown Obsessive Compulsive Scale (Y-BOCS).