An undiagnosed bipolarity may be one of the reasons connected with treatment resistance in major depressive disorder. In recent years, the tools have been introduced helping to assess the bipolarity such as the Hypomania Checklist (HCL-32) and the Mood Disorder Questionnaire (MDQ). The objective of the study was to compare scores of bipolarity measured with these scales between patients with treatment-resistant and treatment non-resistant depression within the framework of all-Poland multi-center study TRES-DEP.

One-thousand and fifty-one patients recruited from 150 outpatient psychiatric clinics in Poland and fulfilling DSM-IV criteria for single or recurrent major depressive episode were studied. Five-hundred and sixty-nine patients were identified as having treatment-resistant depression on account of non-obtaining remission in recent depressive episode after minimum of two courses of adequate treatment with antidepressant drugs. They were compared with 482 patients with non-treatment-resistant depression. All patients were assessed using the structured demographic and clinical data interview, as well as the Polish version of HCL-32 and MDQ

Patients with treatment-resistant depression scored significantly higher compared with non-resistant depression both on HCL-32 and MDQ (11.9+8.3 vs 8.5±7.7 and 4.3±3.5 vs 2.7±2.9, respectively). Independent risk factors of treatment resistance were scoring of14 or higher on HCL-32, and 6 or higher on MDQ.

Higher scoring by patients with treatment-resistant depression on both HCL-32 and MDQ may suggest a possible association between bipolarity and worse response to antidepressant drugs in patients with major depressive disorder.