RESUMEN
BACKGROUND: Health-care workers have an increased risk for chronic low back pain (LBP) leading to reduced workability. Depression, a highly prevalent, costly and disabling condition, is commonly seen in patients with sub-acute LBP. This study investigated the psychometric properties and construct-validity of a modified 9-item Patient Health Questionnaire (PHQ-9-mFIN) in female health-care workers with sub-acute LBP. METHODS: Reliability (internal consistency, test-retest repeatability) was assessed using standard methods. Construct validity of the PHQ-9-mFIN was assessed as level of depressive symptoms (PHQ-9-mFIN: 0-4 none, 5-9 mild, ≥10 at least moderate) against the RAND 36 Health Survey, a valid measure of health-related quality of life (HRQoL). Additionally, the strength of the association between the levels of PHQ-9-mFIN and selected biopsychosocial factors was determined. RESULTS: The internal consistency of the PHQ-9-mFIN was high (Cronbach's α = 0.82) and the test-retest repeatability scores (n = 64) were moderate: Pearson's correlation was 0.73 and Intraclass Correlation Coefficient (ICC) 0.73 (95% CI: 0.58 to 0.82). Construct validity (Spearman correlation) against the Physical and Mental component items and their summary scales of the RAND 36 were much higher for the Mental (range, - 0.40 to - 0.67 and - 0.64) than for the Physical (range, - 0.08 to - 0.43 and - 0.22). There was a clear stepwise association (p < 0.001) between the levels of depressive symptoms and General health (physical component, range, 59.1 to 78.8). The associations with all items of the Mental components were strong and graded (p < 0.001). All participants had low scores for Bodily pain, regardless of the level of depressive symptoms. There was a strong association (p ≤ 0.003) between the levels of PHQ-9-mFIN and multisite pain, lumbar exertion and recovery after workdays, neuromuscular fitness in modified push-ups, workability, and fear of pain related to work. CONCLUSIONS: The PHQ-9-mFIN showed adequate reliability and excellent construct validity among female health-care workers with recurrent LBP and physically strenuous work. TRIAL REGISTRATION: NCT01465698 .