Norwegian reference data on the Fatigue Questionnaire and the Patient Health Questionnaire-9 and their interrelationship

Background Population-based reference data on frequently used questionnaires are important for comparative purposes. Due to changes in health and lifestyles, such data should be updated every other decade. The objectives of this study were to establish Norwegian population-based reference data on the Fatigue Questionnaire (FQ) and the Patient Health Questionnaire-9 (PHQ-9) on depression, to compare the FQ-scores with our previous reference data from 1996, and to explore the relationship between the scores on these two instruments. Methods In 2015, a representative sample of 6,012 Norwegians aged 18–80 years was mailed a questionnaire including the FQ and the PHQ-9, and 36% responded. Complete FQ-scores were delivered by 2,041 subjects, and complete PHQ-9 scores by 2,086 subjects. The scores are displayed according to sex and 10-year age groups. Results Few 2015 mean scores of mental, physical, and total fatigue differed significantly from those of 1996, and the same was found for the prevalence rates of chronic fatigue. The exception was a significantly lower prevalence in 2015 of mean fatigue scores and prevalence of chronic fatigue in females ≥ 60 years. The prevalence of major depressive episode (MDE) based on the PHQ-9 sum score cut-off ≥ 10 was 5.9% for males and 9.8% for females, and 2.5% and 3.8% using a DSM-based algorithm with at least five endorsed criteria including either anhedonia or depressed mood. The correlation between the FQ and the PHQ-9 was 0.59, implying 36% shared variance. Conclusions This study showed considerable interrelationship between the FQ and the PHQ-9 constructs. The reference data show that scores on the FQ have only improved significantly in persons aged 60 or more years between 1996 and 2015. Our prevalence findings of MDE based on the PHQ-9 are in accordance with the findings from other countries. The FQ and the PHQ-9 should be used together in epidemiological and clinical studies.