Background: Two instruments commonly used in primary care research to measure depressive severity are the Patient Health Questionnaire-9 (PHQ-9) and the Hopkins Symptom Checklist-20 (HSCL-20). However, there is little information regarding the relationship between clinical information derived from these scales. The present study investigates the psychometric properties of the PHQ-9 and HSCL-20, determines the degree of instrument concordance, and describes the factor structure of the HSCL-20.
Methods: A secondary data analysis from a randomized controlled trial was performed. A total of 405 primary care patients with major depressive disorder and/or dysthymia were administered the PHQ-9 and the HSCL-20 when recruited for the study.
Results: Good internal consistency reliability estimates were obtained for both scales (PHQ-9 alpha=0.803; HSCL-20 alpha=0.837). All PHQ-9 inter-item and corrected item-total correlations showed that no item detracted from overall scale functioning. HSCL-20 items assessing overeating, poor appetite, and sexual interest were poorly correlated with other items and with the total scale score. A positive, moderate strength relationship was found between the instruments (r=0.54, p<0.0001). Exploratory factor analysis of the HSCL-20 yielded a six-factor structure, which accounted for almost 63% of the variance in total score. The largest contribution to common variance in the scale was provided by an "anxiety and self-reproach" factor.
Conclusions: PHQ-9 and HSCL-20 total scores were moderately correlated. Although the HSCL-20 is utilized as a measure of depression severity, it may lack sufficient specificity to be an accurate reflection of depression status per se.