The AUSCAN subscales, AIMS-2 hand/finger subscale, and FIOHA were not unidimensional scales.
OBJECTIVES
Evaluate the internal construct validity of the Australian/Canadian (AUSCAN) index for hand osteoarthritis (HOA) and identify the physical function instrument with best performance.


STUDY DESIGN AND SETTING
AUSCAN, AIMS-2 (Arthritis Impact Measurement Scale 2), and Functional Index of HOA (FIHOA) were self-completed by 209 HOA patients (mean [standard deviation] age 61.6 [5.7] years) at baseline and 128 at follow-up. Rasch analysis was performed.


RESULTS
AUSCAN pain, physical function, and stiffness subscales comprised three constructs. AUSCAN scale performance was improved after removal of "Pain at rest" from the pain scale and division of physical function into two scales of high precision and grip strength tasks. AIMS-2 hand/finger subscale and FIHOA were improved after removal of one and two items, respectively and collapse of two AIMS-2 response categories. AUSCAN physical function scale showed better targeting to the sample and higher person reliability compared with FIHOA and especially AIMS-2 because of less "severe" items concerning grip strength tasks as opposed to precision tasks.


CONCLUSION
The AUSCAN subscales, AIMS-2 hand/finger scale, and FIHOA were not unidimensional. However, deletion of misfitting items improved scale performance. The revised AUSCAN physical function and FIHOA scales are preferable for measurement of grip strength and precision tasks, respectively.