Depression in children and adolescents is far from infrequent or inconsequential (Birmaher et al., 1996; Lewinsohn et al., 1998; Merikangas et al., 2010; Rohde et al., 2012; Thapar et al., 2012). Even though many children recover, early-onset depression remains a potent risk factor for subsequent mental health problems and other negative outcomes (Harrington et al., 1990; Weissman et al., 1999; Kovacs et al., 2016). In addition, early gender differentiation has also been observed, where adolescent girls tend to have both elevated symptoms and a different developmental course of depression (Hankin et al., 1998; Twenge and Nolen-Hoeksema, 2002; Dunn and Goodyear, 2006; Dekker et al., 2007; Essau et al., 2010). Accurately detecting early depression symptoms, and detecting them accurately for boys and girls is thus a public health imperative. In that regard, brief self-reports may be especially useful, as such screeners can rapidly identify “at-risk” youth in need of further evaluation and possibly treatment.

This may especially be true for Norway, where the 2014 Public Health report states that “mental disorders are a major health problem for children and adolescents in Norway today” (Norwegian Institute of Public Health, 2014). Despite these clearly identified issues and the general focus on adolescent health and development, the use of diagnostically-informative measures in research practice has been somewhat limited in Norway. For example, multiple Norwegian studies investigating early depression (Sund et al., 2003, 2011; Lundervold et al., 2013; Larsson et al., 2016) have utilized various versions of the Mood and Feelings Questionnaire (MFQ; Angold et al., 1995). Its wide use in studies of developmental epidemiology notwithstanding, the MFQ is fairly extensive (34 items for the full and 13 items for the short version), rendering it not necessarily the best brief instrument. And even though the MFQ can theoretically aid in clinical screening, its cut-offs are not fully established or necessarily even recommended¹. Other reports have used shorter and thus more practical instruments; for example the 12-item and 5-item Symptom Checklists (SCL-12, and SCL-5) (Heyerdahl et al., 2004; Derdikman-Eiron et al., 2012; Myklestad et al., 2012), and a 6-item Depressed Mood Inventory (Wichstrøm, 1999; Abebe et al., 2016), all of which appear to measure anxiety and depression and to have been derived from the 25-item Hopkins Symptom Checklist (HSCL) for adults (Derogatis et al., 1974). However, it is not entirely clear how appropriate these derivations may be for the assessment of adolescents, or how well they differentiate between anxiety and depression given that they tend to conflate the two into “anxiety-depression” (Heyerdahl et al., 2004) and are meant to measure general “psychological” or “global mental distress” (Tambs and Moum, 1993; Strand et al., 2003; Myklestad et al., 2012). In addition, the items, responses, cut-off scores, and clinical interpretation of these HSCL derivations appear to vary from study to study in Norway (i.e., from version to version; Heyerdahl et al., 2004; Derdikman-Eiron et al., 2012; Abebe et al., 2016). Finally, psychiatric symptoms among Norwegian adolescents have also been evaluated using the Strengths and Difficulties Questionnaire (SDQ) and its various subscales (Rønning et al., 2004; Indredavik et al., 2005; Goodman et al., 2011). Despite its many advantages and widespread international use, the 25-item SDQ is also somewhat long, and it ideally requires the children's, parental, and teachers' reports for a complete evaluation. Thus, any potential large-scale screening based on multiple SDQ reports/informants may be both impractical and costly. The use of SDQ for such purposes may additionally be questionable, given the related cultural considerations (Heiervang et al., 2007, 2008), as well as its somewhat limited ability to detect mental health disorders in general (Brøndbo et al., 2011), and depressive symptomatology in particular. For example, it is not clear how well the SDQ Emotional Problems subscale differentiates between anxiety and depression, as both classes of problems appear to be assessed under the general heading of “emotional” disorders (Goodman et al., 2000a,b).

Thus, the public health initiatives and the related research and clinical practice in Norway could benefit from a self-report instrument designed to swiftly and effectively screen specifically for early-onset depression based on the common and internationally-validated criteria. The 9-item Patient Health Questionnaire (PHQ-9) adolescent version may be especially well-suited for such purposes (Kroenke et al., 2001; Johnson et al., 2002). First, the PHQ-9 has only 9 items, and it requires only the youth self-reports. The instrument was originally developed to assess symptoms of depression in accordance to the Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV) criteria, but it also corresponds to the more recent DSM-5 criteria (American Psychiatric Association, 2016). Further, the PHQ-9 has been shown to be a valid tool in detecting depression among adolescents across various cultures and settings (Adewuya et al., 2006; Richardson et al., 2010; Fatiregun and Kumapayi, 2014; Tsai et al., 2014). Most importantly, the PHQ-9 also captures depression severity, as it provides both continuous scores and clinically-meaningful classification of depressive symptomatology. Availability of continuous scores ensures no loss of variability, and thus may be both recommended and preferred in research, whereas categorical classification may be more meaningful in clinical practice and assessment. Ideally, a self-report instrument should serve both purposes, and the PHQ-9 for adolescents theoretically does so at the international level. For example, the PHQ-9 was successfully used to estimate the prevalence of Major Depressive Disorder (MDD) in Chinese and Nigerian school samples (Fatiregun and Kumapayi, 2014; Tsai et al., 2014), in addition to the US community and primary care samples (Richardson et al., 2010; Rhew et al., 2016). Such apparent cultural robustness strengthens the potential advantages of this instrument in multiple large scale national and cross-cultural comparisons.

In conclusion, there is a need for a brief and internationally validated instrument that captures both the severity and corresponding clinical categories of self-reported depressive symptomatology in Norwegian adolescent populations. Such an instrument would strengthen both the research and clinical practice in Norway, while simultaneously opening the door to cross-cultural and cross-national monitoring, evaluation, and comparison. Perhaps the PHQ-9—with its brief format, simple self-report administration, and attractive scoring features—can fill this need. This report offers the first step in that direction, by examining the PHQ-9 basic properties and correlates in a sample of Norwegian adolescents. Specifically, we examined: (1) the basic psychometric properties of the PHQ-9, including measurement equivalence (invariance) by gender, and (2) severity and correlates of depressive symptoms among Norwegian middle- and high-school students as measured by the PHQ-9.

Our results appear consistent with previous international reports examining depression in adolescent school samples using the PHQ-9 instrument, including the school samples of Chinese and Nigerian adolescents (prevalence of moderately severe/severe depression = 5.2 and 5.1%, respectively; Fatiregun and Kumapayi, 2014; Tsai et al., 2014) and the community samples of American adolescents (moderately severe/severe depression prevalence = 6.5%, even though this study used a somewhat different clinical classification algorithm; Rhew et al., 2016). Most importantly, our estimate of approximately 6% prevalence of clinically-elevated symptoms is relatively congruent with other recent reports of current depressive symptomatology among Norwegian adolescents, ranging from 2.6% for MDD and 6.3% for depressive disorder not otherwise specified during the 2-month window (Sund et al., 2011), to the 11% prevalence of the less specific “high depressive symptoms” during the past week (Abebe et al., 2016). Nevertheless, these estimates should be interpreted as preliminary given that we used the cut-off scores established internationally but not in Norway, and the sample which was not necessarily representative of all Norwegian adolescents.

Our CFA results confirmed that the PHQ-9 measures adolescent depression as a unidimensional theoretical construct. Relatively high factor loadings (i.e., all loadings ≥0.40) and solid fit indices were observed for the entire sample, and for boys and girls separately. One item (e.g., “Moving or speaking so slowly that other people could have noticed”) consistently showed poorer, yet still acceptable performance. It is possible that the wording and/or meaning were difficult for younger participants to comprehend, as evident in the relatively high number of missing responses on this particular item. Specifically, this question asked about what other people could have noticed, which may be confusing for youngest participants. Refinement or alternative wording of this item may improve the scale performance.

In agreement with previous evidence that girls tend to exhibit greater depressive symptomatology than boys starting around the age of 13 (Hankin et al., 1998; Wichstrøm, 1999; Twenge and Nolen-Hoeksema, 2002), and with other Norwegian reports demonstrating substantive gender differences in measurements of adolescent depression (Lundervold et al., 2013), girls from our sample appeared to have greater depression problems. However, we did not proceed to perform formal tests of gender differences because we could not fully establish metric or scalar equality for the PHQ-9 instrument across genders. This means that girls and boys cannot be directly compared in terms of the PHQ-9 scores without reservations, contrasting previous international reports of the PHQ-9 measurement invariance by gender and age (Yu et al., 2012; Petersen et al., 2015). However, our results may not be out of place in a Norwegian context, where only partial measurement-equivalence for gender was reported for the SDQ instrument in adolescent populations (Bøe et al., 2016). Clearly, the causes and implications of these non-equivalence results should be investigated further when it comes to the use of PHQ-9 (and possibly other instruments for which measurement equivalence was not fully tested before use) in Norway. Nevertheless, it should also be noted that these findings do not preclude utilization of the PHQ-9 in research practice, as long as the analyses are stratified by gender (Bøe et al., 2016).

Finally, we examined divergent and convergent validity, as well as the associations between the PHQ-9 scores for boys and girls separately. Given that the PHQ-9 was not previously used in Norway, we only had general expectations that the direction and magnitude of these associations would be comparable to those from other Norwegian studies on adolescent depression. This was generally the case. We found no strong evidence for the association between depression in adolescents and their basic demographic characteristics such as religion or immigrant status. Such a pattern may be reflecting the somewhat inconclusive state of knowledge regarding the mental health among adolescent immigrants in Norway (Abebe et al., 2014). Other putative risk factors—including age, not living together with both biological parents, and diagnosed chronic illness—were associated with depressive symptomatology differently for boys and girls. Older age appeared to be a risk factor for depression among girls only, while not living with both biological parents or chronic illness appeared to be risk factors only among boys. In contrast, high number of moves and school changes, perceptions of one's family as poor by Norwegian standards, social isolation and lack of close friendships, history of suicide attempts and self-harms, and elevated SDQ emotional problems were uniformly and significantly associated with elevated depression for both genders. Similar patterns—for example, the associations between elevated depression and residential instability, low SES, not living with both biological parents, school dissatisfaction and lack of close friendships (Sund et al., 2003; Myklestad et al., 2012)—were observed in other Norwegian adolescents samples. More importantly, our results demonstrated the high convergence between the PHQ-measures with other theoretically related constructs such as suicidality or impairments in emotional adjustment (Goodman and Goodman, 2009; Hawton et al., 2012; Silverstone et al., 2015), where the standardized regression coefficients with the SDQ Emotional problems scores exceeded 0.5 for both genders. They also suggest the potential effectiveness of the PHQ-9 in preliminary identification of youth at high risk for depression using the large-scale epidemiological surveys.

The current study is limited by several factors, including its convenience sample and reliance on self-reports for all indicators. We also used a somewhat shorter time reference and assessed the PHQ-9 depressive symptomatology during the last 7 days as recommended by the American Psychiatric Association (2016). Most importantly, our adolescent PHQ-9 self-reports were not validated against the external diagnostic criteria, such as the official psychiatric diagnoses for example. However, it should be noted that the PHQ-9 has been internationally validated against various diagnostic interviews in multiple adolescent studies (Richardson et al., 2010; Allgaier et al., 2012; Ganguly et al., 2013; Tsai et al., 2014), with some reports even using the PHQ-9 itself as a gold standard against which to validate other measures of depression in community youth samples (Rhew et al., 2016). Nevertheless, external validation of the PHQ-9 against diagnostic interviews would have strengthened our study, especially because we used somewhat conservative criteria and because there may be cultural, ethnic, and national variations in depressive symptomatology and corresponding clinical cut-offs (Kroenke and Spitzer, 2002; Richardson et al., 2010; Allgaier et al., 2012; Ganguly et al., 2013; Jaber et al., 2015). Full validation, including the validation of the originally suggested cut-offs and associated diagnostic categories, is therefore necessary before any clinical evaluation is to be undertaken using the PHQ-9 among Norwegian youth.

Despite these limitations, the PHQ-9 appears to be a promising research tool, potentially offering clinically-relevant classification of adolescent depressive symptomatology in addition to the symptom severity captured by continuous scores. Most importantly, given the internationally validated and streamlined clinical criteria, the use of PHQ-9 has the potential to advance not only national, but also cross-national assessments and comparisons of depressive symptomatology among youth populations. As such, we encourage further exploration of the PHQ-9 adolescent instrument in developmental epidemiology research and in studies of general adolescent health and development. Nevertheless, further study of the hereby observed measurement non-equivalence, as well as a comprehensive validation against the proper diagnostic criteria are required before the PHQ-9 is to be used for youth psychiatric screening in Norway.

This study was carried out in accordance with the recommendations of the Norwegian Centre for Research Data (NSD; http://www.nsd.uib.no/nsd/english/pvo.html; Protocol#39513) with informed consent from all participants or guardians.

The present report was drawn from a larger adolescent development project, directed by JBA and GSB. Both JBA and GSB contributed to study design, development of research questions, data analyses, and writing. Both authors approved the final manuscript.