RESUMEN
BACKGROUND: The Patient Health Questionnaire-9 (PHQ-9) has been widely used in research and clinical settings. To be able to attribute differences in PHQ-9 scores between groups with different cultural backgrounds to differences in the level of depression, the instrument has to possess measurement invariance. METHODS: Data from the Apollo-D study were used. We used two strongly contrasting cultural groups (n=1772). Measurement invariance was assessed by comparing four categorical single factor models with an increasing number of restrictions, representing an increasingly stronger measurement invariance assumption. RESULTS: The PHQ-9 was measurement invariant for ethnicity in women and partially measurement invariant for ethnicity in men. The item 'psychomotor problems' seemed to be culturally biased in the Surinam Dutch males. It had a higher loading and threshold compared to Dutch males. LIMITATIONS: The sample is restricted to high risk primary care patients, we did not include a gold standard measure of depression and the analyses pertain to a single cross cultural comparison. CONCLUSIONS: The observed higher total depression score for females in the Surinam Dutch group can be attributed to a true difference in the latent trait depression. For Surinam Dutch and Dutch men some caution is warranted when comparing results obtained with the PHQ-9. In the former group the scores may be biased slightly downward. Future research is needed to examine how the item 'psychomotor problems' performs in different populations. These findings highlight the necessity of establishing measurement invariance before drawing conclusions based on observed scores.