Major depressive disorder (MDD) is one of the most prevalent mental disorders worldwide, affecting between 4% and 10% of the population,^{1,Reference Arias-de la Torre, Vilagut, Ronaldson, Bakolis, Dregan and Martín2} and is among the main causes of disability, dependency, loss of quality of life and health expenditure. Owing to the high prevalence of MDD, the use of depression screening programmes has been proposed as a key strategy for reducing its burden and impact. These strategies focus on early identification of possible MDD cases to prevent onset, i.e. to reduce the frequency of missed diagnoses. In addition, in cases where the disorder is active, they could help prevent its progression to more severe or chronic stages.

The main context in which screening strategies for depression have been implemented is primary care.^{Reference Arias de la Torre, Ronaldson, Vilagut, Martínez-Alés, Dregan and Bakolis3} As suggested by previous research,^{Reference Thombs, Markham, Rice and Ziegelstein4} if case detection relies on valid and reliable screening measures, such as the eight- or nine-item versions of the Patient Health Questionnaire (the PHQ-8 and PHQ-9, respectively), and possible cases are followed up by a specialist, the use of screening programmes could be considered to be an effective, efficient and feasible strategy for detection of MDD and reduction of its impact.^{Reference Arias de la Torre, Ronaldson, Vilagut, Martínez-Alés, Dregan and Bakolis3} However, in primary care, systematic screening for MDD can only be recommended if a follow-up clinical consultation can be guaranteed for all that need it (i.e. in contexts with enough healthcare resources to assess, diagnose and, if required, recommend and deliver evidence-based treatment).^{Reference Thombs, Markham, Rice and Ziegelstein4} Of note, the effectiveness of screening as a preventive measure for MDD could be seriously compromised in the absence of engagement with relevant mental health services after a positive result.

According to the National Institute for Health and Care Excellence guidelines for depression,⁵ the first step after a positive screening result must be a comprehensive clinical assessment to diagnose MDD or rule out possible false positives. After this assessment, a treatment should be agreed and delivered by a specialist for those with MDD. Starting medical consultation using structured and semi-structured clinical interviews for ruling out false positives could optimise the use of healthcare resources. This assessment would also provide an opportunity to improve the engagement of patients with healthcare services by giving them further information about the assessment and treatment options.

In addition to primary care settings, the implementation of systematic screening for MDD has been proposed in community contexts (i.e. non-clinical and population-based).^{Reference Arias de la Torre, Ronaldson, Vilagut, Martínez-Alés, Dregan and Bakolis3} Although high-quality evidence supporting the implementation of systematic screening in these settings is still very sparse, its potential relevance as a preventive strategy should be noted, particularly among population groups with limited access to or use of healthcare resources, such as homeless people. Furthermore, some studies suggest that the data derived from community screening strategies could enhance monitoring of MDD at the population level by synergistically enriching these data with clinical data, such as variables related to use and engagement with health services from electronic health records.^{Reference Arias de la Torre, Ronaldson, Vilagut, Martínez-Alés, Dregan and Bakolis3} However, and similar to primary care, community-based strategies would generate high costs, as subsequent clinical attention for participants screening positive would also be required. These issues limit community-based strategies to contexts with sufficient social and healthcare resources and their potential impact only to participants who have access to and are willing to attend to a consultation in the case of a positive screening result, i.e. engagement with mental health services.

One relevant aspect to consider in relation to MDD screening, in both primary care and community settings, is the inevitable proportion of false positives derived from the use of screening questionnaires.^{Reference Arias de la Torre, Ronaldson, Vilagut, Martínez-Alés, Dregan and Bakolis3} A solution suggested to deal with this issue is the use of a higher (more conservative) cut-off point on screening questionnaires, which increases their specificity and, consequently, reduces the number of false positives (e.g. a cut-off point of 12 instead of 10 for the PHQ-8 and PHQ-9). It should also be noted that even false positives obtained with less conservative cut-off scores should ideally receive clinical attention as some may become cases later on; they thus constitute a potentially relevant focus for MDD early prevention strategies. In addition, self-screening via the internet and selective strategies focusing on high-risk population groups have been proposed^{Reference Houston, Cooper, Vu, Kahn, Toser and Ford6} to maximise the feasibility and impact of screening for MDD. Such selective strategies address vulnerable groups, such as pregnant women, patients with multiple long-term conditions or people with reduced social support.^{Reference Arias de la Torre, Ronaldson, Vilagut, Martínez-Alés, Dregan and Bakolis3,Reference Woody, Ferrari, Siskind, Whiteford and Harris7} However, similar to the other screening programmes, a subsequent clinical consultation should be available with these approaches. Besides, although evidence of the effectiveness and implementation of self-screening and selective strategies is still sparse, ensuring follow-up engagement with relevant mental health services for those who need it would constitute an indirect way to improve the effectiveness of outcomes of MDD screening.

Various studies have looked for factors that could predict engagement with mental health services following a positive depression screening result.^{Reference Cohan, Waxmonsky, Fogel, Pradhan and Sekhar8,Reference Löwe, Scherer, Braunschneider, Marx, Eisele and Mallon9} In addition to availability and accessibility of specialised care, the information received during the screening process was identified as a key factor for ensuring engagement with mental health services when required.^{Reference Cohan, Waxmonsky, Fogel, Pradhan and Sekhar8,Reference Löwe, Scherer, Braunschneider, Marx, Eisele and Mallon9} Moreover, it has been suggested that the information offered to participants during the screening process might have a positive effect on their symptom management and their willingness to attend mental health services. Although further research is needed, the evidence available shows promising results regarding the benefits of actively involving participants in the screening process by giving them feedback about the process itself and its outcomes and, when needed, offering them a follow-up consultation.^{Reference Cohan, Waxmonsky, Fogel, Pradhan and Sekhar8,Reference Löwe, Scherer, Braunschneider, Marx, Eisele and Mallon9} In addition, it opens a window of opportunity for enhancing the effectiveness of screening strategies for MDD, particularly for selective strategies focused on the most vulnerable population groups. Thus, the systematic incorporation of specific and targeted feedback as a key component of screening strategies for MDD would be a helpful resource to enhance their effectiveness as preventive tools and, consequently, to reduce the impact and burden of MDD at both individual and population levels.