A differential item function analysis of somatic symptoms of depression in people with cancer

Background: The overlap of somatic symptoms of depression with symptoms of cancer treatment is widely acknowledged and studied. However, this literature provides little guidance for clinicians as to whether these items should be used in assessing depression. The current study examined the appropriateness of using somatic items for assessment of depression in people with cancer.

Methods: People with newly diagnosed breast, lung or colorectal cancer (n=251) completed the Patient Health Questionnaire-9 (PHQ9) shortly after cancer diagnosis but before cancer treatment (baseline), 4 months later, typically during or shortly after treatment, and 12 months later. Pharmacy data was used to classify participants as having low somatic symptoms or high somatic symptoms. Differential item function (DIF) compared the functioning of the somatic items of the PHQ9 in the low vs. high symptom groups and the chemotherapy vs. no chemotherapy groups at the 4-month assessment.

Results: Significant DIF was not found on any of the four somatic items of the PHQ9 and differences in the item parameters of the somatic items were not consistent across the groups. However, fatigue and sleep indicated only mild depression. Only removing the fatigue item greatly affected the number screening positive for depression at 4 months (8.3%) but removing the other somatic items did not have as large an effect. Only one participant at baseline screened positive for depression by somatic symptoms alone (no psychological symptoms) and no participants screened positive by somatic symptoms alone at 4 months and 12 months.

Limitations: The sample size was small for DIF and consisted of mostly women with breast cancer.

Conclusions: Somatic symptoms of depression can continue to be administered to people with cancer, however the fatigue and sleep items should be used with caution.