Background:
This study investigates whether an analysis, based on Item Response Theory (IRT), can beused for initial evaluations of depression assessment instruments in a limited patient samplefrom an affective disorder outpatient clinic, with the aim to finding major advantages anddeficiencies of the instruments.
Methods:
Three depression assessment instruments, the depression module from the Patient HealthQuestionnaire (PHQ9), the depression subscale of Affective Self Rating Scale (AS-18-D) andthe Montgomery-Asberg Depression Rating Scale (MADRS) were evaluated in a sample of61 patients with affective disorder diagnoses, mainly bipolar disorder. A ‘3- step IRTstrategy’ was used.
Results:
In a first step, the Mokken non-parametric analysis showed that PHQ9 and AS-18-D hadstrong overall scalabilities of 0.510 [C.I. 0.42, 0.61] and 0,513 [C.I. 0.41, 0.63] respectively,while MADRS had a weak scalability of 0.339 [C.I. 0.25, 0.43]. In a second step, a Raschmodel analysis indicated large differences concerning the item discriminating capacity andwas therefore considered not suitable for the data. In third step, applying a more flexible twoparameter model, all three instruments showed large differences in item information anditems had a low capacity to reliably measure respondents at low levels of depression severity.
Conclusions:
We conclude that a stepwise IRT-approach, as performed in this study, is a suitable tool forstudying assessment instruments at early stages of development. Such an analysis can giveuseful information, even in small samples, in order to construct more precise measurementsor to evaluate existing assessment instruments. The study suggests that the PHQ9 and AS-18-D can be useful for measurement of depression severity in an outpatient clinic for affectivedisorder, while the MADRS shows weak measurement properties for this type of patients.