Background information
Approximately 2 percent of the Dutch population suffers from Obsessive compulsive disorder (OCD). Unfortunately, little is known about the long term prognosis of OCD and the predictors of a chronic course. Consequently, there is a lack of understanding the reasons why some patients do not respond positively to treatment. Considering the high healthcare costs and the impairment at work and social life, comprehensive research is crucial in reducing these problems.

NOCDA is a multicentre cohort study in which a large number of OCD patients participate. The patients will be monitored for several years in order to study the psychological, biological, and social determinants of a therapy resistant and therefore chronic course of OCD. The outcomes of NOCDA can aid therapists in recognising these predictive factors and contributes to the development of preventive measures. In summary, this study aims to improve mental healthcare for patients with OCD, especially those that do not benefit from the existing therapies.

Embedded research
This study takes advantage of an already existing infrastructure for longitudinal research and clinical studies; that of the AMsterdam Study to Anxiety and Depression (AMSTAD), founded in 2002. AMSTAD is the result of a joint effort by the VU medical centre’s psychiatric department and the outpatient clinics of GGZ inGeest. Research assistants, data managers, methodologists, epidemiologists, psychiaters, and clinical researchers work together within this infrastructure to study disorders within the realm of mental health. On top of that, the current  Netherlands OCD Association (NOCDA) was designed in close collaboration with the well-established Netherlands Study to Depression and Anxiety (NESDA).

Study population
The NOCDA study includes 419 participants over 18 years who are diagnosed with OCD. To ensure the diagnosis, we used the Structural Diagnostic Interview for DSM-IV disorders (SCID I; First, Spitzer, Gibbon, & Williams, 1999). Every single patient that registered for OCD treatment during the inclusion period, at any of the partaking mental health institutions, was asked to participate. In addition, all patients with ongoing treatments were asked to participate, regardless of the stage of their disorder, the exact symptoms they experienced, and existing comorbidity. This results in a diverse group of participants in which some people are being treated at the outpatient wards whereas others are admitted in psychiatric hospitals. In addition, there is variation in the duration of symptoms and the history of previous treatments.

Measurement tools

During the first measurements (baseline), participants were interviewed extensively by trained research assistants who used validated semi-structured interviews. This took about 5 to 6 hours per interview, including the time needed to complete additional self-report questionnaires. Preferably, participants were interviewed at the location of treatment. However, participants who were not capable of visiting could stay at home and the research assistant would visit them for the baseline measurements. One year after the first measurements, all participants received a self-report questionnaire. 

Interviews by the research assistants were repeated 2, 4 and 6 years after baseline. Although still extensive they were less time-consuming and lasted 3 to 4 hours. These interviews provide insight in the severity of OCD, comorbidity, general well-being, quality of life, influence of OCD on daily life, socio-demographics (e.g. socioeconomic status), social relations and support, (extended) family composition, and other possible predictors such as life events, medical consumption (medical costs and medication use), alcohol and drug abuse.

In addition, biological measurements are part of the study. During every interview, participants’ weight, hip-waist ratio, and blood pressure will be recorded. Blood samples are taken at the baseline and the 6 years post-baseline measurements. This will be used for genetic research, among others.