Purpose: We studied the validity and responsiveness of an item response theory (IRT) scoring method for assessing major depressive episode (MDE) and generalized anxiety disorder (GAD) severity based on direct assessment of DSM-IV-TR symptoms.
Methods: Prospective cohort study (baseline, 1-month, 3-months assessments) of patients seeking help for incident or aggravated mood or anxiety symptoms from primary, outpatient and inpatient mental health centers (N = 244; 67.81 % active cases - 100 % under psychiatric treatment). The drop-out rate at 3 months was 24.89 %. Patients were assessed at each follow-up for presence/absence of DSM-IV symptoms of MDE (nine symptoms) and GAD (eight symptoms). IRT scores for depression (INS-D) and anxiety (INS-G), based on response patterns, were obtained by means of a 2-parameter model. Diagnostic accuracy was assessed with receiver operating characteristic analysis, using a blinded MINI interview as gold standard. Scores' construct validity was compared with external clinician-administered (Hamilton Depression Rating Scale, HRSD; Hamilton Anxiety Rating Scale, HAM-A) and self-reported severity measures (PHQ-9; Beck Anxiety Inventory-Subjective Aspects, BAI-Sub). Responsiveness was analyzed based on the evolution of HRSD and HAM-A scores.
Results: Both severity scores showed excellent reliability (INS-D: 0.92; INS-G: 0.93) and yielded high diagnostic accuracy (INS-D: AUC = 0.96; INS-G: AUC = 0.91) with respect to MINI diagnoses. INS-D and INS-G had higher correlations with clinician-administered measures of the same disorder (INS-D-HRSD: 0.73; INS-G-HAM-A: 0.53) than with self-reported measures (INS-D-PHQ-9: 0.69; INS-G-BAI-Sub: 0.49). Patients who recovered during follow-up showed important decreases in severity (Cohen's d INS-D:-1.38; INS-G: -1.75). About 90 % variance of INS-D and INS-G score changes over time was associated with changes in clinical status.
Conclusions: INS-D and INS-G are short reliable, valid, and responsive measures that can be used for diagnostic and severity assessment of mood and anxiety disorders in outpatient care.