Reliability and cross-country equivalence of the 8-item version of the Patient Health Questionnaire (PHQ-8) for the assessment of depression: results from 27 countries in Europe

BACKGROUND: The 8-item version of the Patient Health Questionnaire (PHQ-8) is one of the self-reported questionnaires most frequently used worldwide for the screening and severity assessment of depression. However, in some European countries its reliability is unknown, and it is unclear whether its psychometric properties vary between European countries. Therefore, the aim of this study was to assess the internal structure, reliability and cross-country equivalence of the PHQ-8 in Europe. METHODS: All participants from the 27 countries included in the second wave of the European Health Interview Survey (EHIS-2) between 2014 and 2015 with complete information on the PHQ-8 were included (n = 258,888). The internal structure of the PHQ-8 was assessed using confirmatory factor analyses (CFA) for categorical items. Additionally, the reliability of the questionnaire was assessed based on the internal consistency, Item Response Theory information functions, and item-discrimination (using Graded Response Models), and the cross-country equivalence based on multi-group CFA. FINDINGS: The PHQ-8 shows high internal consistency for all countries. The countries in which the PHQ-8 was more reliable were Romania, Bulgaria and Cyprus and less reliable were Iceland, Norway and Austria. The PHQ-8 item with highest discrimination was item 2 (feeling down, depressed, or hopeless) in 24 of the 27 countries. Measurement invariance between countries in Europe was observed from multigroup CFA at the configural, metric and scalar levels. INTERPRETATION: The results from our study, likely the largest study to the date assessing the internal structure, reliability and cross-country comparability of a self-reported mental health assessment measure, shows that the PHQ-8 has an adequate reliability and cross-country equivalence across the 27 European countries included. These results highlight the suitability of the comparisons of the PHQ-8 scores in Europe. They could be helpful to improve the screening and severity assessment of depressive symptoms at the European level. FUNDING: This work was partially funded by 10.13039/100006301CIBER Epidemiology and Public Health (CIBERESP) as part of the Intramural call of 2021 (ESP21PI05).