Lawton IADL scale in dementia: can item response theory make it more informative?

BACKGROUND: impairment of functional abilities represents a crucial component of dementia diagnosis. Current functional measures rely on the traditional aggregate method of summing raw scores. While this summary score provides a quick representation of a person's ability, it disregards useful information on the item level.

OBJECTIVE: to use item response theory (IRT) methods to increase the interpretive power of the Lawton Instrumental Activities of Daily Living (IADL) scale by establishing a hierarchy of item 'difficulty' and 'discrimination'.

METHODS: this cross-sectional study applied IRT methods to the analysis of IADL outcomes. Participants were 202 members of the Scottish Dementia Research Interest Register (mean age = 76.39, range = 56-93, SD = 7.89 years) with complete itemised data available.

RESULTS: a Mokken scale with good reliability (Molenaar Sijtsama statistic 0.79) was obtained, satisfying the IRT assumption that the items comprise a single unidimensional scale. The eight items in the scale could be placed on a hierarchy of 'difficulty' (H coefficient = 0.55), with 'Shopping' being the most 'difficult' item and 'Telephone use' being the least 'difficult' item. 'Shopping' was the most discriminatory item differentiating well between patients of different levels of ability.

CONCLUSIONS: IRT methods are capable of providing more information about functional impairment than a summed score. 'Shopping' and 'Telephone use' were identified as items that reveal key information about a patient's level of ability, and could be useful screening questions for clinicians.