Background: Depressive symptoms are common in rheumatic diseases and influence patients' quality of life. The Patient Health Questionnaire-9 (PHQ-9), which assesses symptoms of depression, is valid in English in patients with systemic sclerosis (SSc). However, the measurement properties of the PHQ-8 (short version of the PHQ-9) have not been evaluated in Swedish patients with SSc.
Objective: To investigate different aspects of validity and reliability of the PHQ-8 in Swedish (PHQ-8 Swe) for individuals with SSc.
Methods: A total of 101 patients with SSc participated. Content validity was evaluated via interviews of 11 patients and 10 health professionals. Construct validity, internal consistency test-retest reliability, and floor/ceiling effects were evaluated in 90 patients.
Results: Content validity was satisfactory, but some linguistic adjustments were made. Confirmatory factor analysis supported a better fit for a two-factor structure. Moderate-to-strong correlations were found between the PHQ-8 Swe and scleroderma HAQ including VAS (rs = 0.4-0.7); Multidimensional Assessment of Fatigue (rs = 0.7); RAND-36 subscales (rs = - 0.5 to - 0.8); and lung disease severity (Medsger scores) (rs = 0.4). There were weak correlations (rs = <0.4) between the PHQ-8 Swe and modified Rodnan skin score; and vascular, heart, and kidney disease severity. Cronbach's alpha was 0.85, corrected item-to-total correlations were >0.40, and the ICC for the total score was 0.83. No floor/ceiling effects were found.
Conclusion: The PHQ-8 Swe has satisfactory content validity and sufficient reliability in patients with in majority limited SSc. It is more strongly associated with self-reported disability, pain, disease interferences with daily activities, fatigue, and quality of life than with disease severity, except for a moderate association with lung severity.
Keywords: Depression; Patient-reported outcome measures; Psychometrics; Quality of life; Scleroderma systemic.