Rasch analysis of the Western Ontario MacMaster questionnaire in 2205 patients with osteoarthritis, Rheumatoid Arthritis (RA), & fibromyalgia (FM)

OBJECTIVE: Advances in health measurement have led to the
application of Rasch Item Response Theory (IRT) analysis
(Rasch analysis) to evaluate instruments measuring health
status and quality of life of patients, including the Health
Assessment Questionnaire and SF-36. This study investigated
the extent to which the Western Ontario MacMaster
osteoarthritis questionnaire (WOMAC) satisfies the Rasch
model, particularly in respect to unidimensionality, item
separation, and linearity.

METHODS: The study included a total
of 2205 patients, 1013 with rheumatoid arthritis (RA), 655
with osteoarthritis of the knee or hip (OA), and 537 with
fibromyalgia. All patients completed the WOMAC as part of a
longitudinal study of rheumatic disease outcomes. To examine
whether the WOMAC pain and function scales each fits the Rasch
model, the Winsteps program was used to assess item
difficulty, scale unidimensionality, item separation, and

RESULTS: Although the WOMAC worked best in OA,
regardless of disorder, both the pain and function scales were
unidimensional, had adequate item separation, and had a long
range (25-150) of linearity in the function scale. Several
functional items, however, had a high information weight fit
(INFIT) statistic, indicating poor fit to the model. These
items included "getting in and out of the bath" and "going
down stairs."

CONCLUSION: The WOMAC generally satisfies the
requirements of Rasch item response theory across all
disorders studied, and is an appropriate measure of lower body
function in OA, RA and fibromyalgia. Although some individual
items do not fit well, it is not likely that removing such
items would result in more than overall minimal differences,
and it will be difficult to remove traces of
multidimensionality while keeping the central constructs of
progressive lower body musculoskeletal abnormality intact. In
addition, it is possible that a "purer", still more
unidimensional instrument would be less useful in clinical
trials and epidemiological studies by restricting the range of
the scale.

Wolfe F, Kong SX

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