Measurement properties of the SF-12 health survey in Parkinson's disease.

The 12-item Short-Form Health Survey (SF-12) is an abbreviated version of the SF-36, one of the most widely used patient-reported health outcome rating scales. Similar to the SF-36, it yields summary scores of physical and mental health (PCS and MCS, respectively). However, SF-36 derived PCS and MCS scores have not been found valid in neurological disorders such as Parkinson's disease (PD). Here we used modern psychometric methodology (Rasch analysis) to test the SF-12 in PD, and explored the appropriateness of a total SF-12 score representing overall health. SF-12 data from 150 non-demented people with PD (56% men; mean age/PD-duration, 70/5 years) were analyzed regarding Rasch model fit for the PCS, MCS, as well as for the full SF-12. Data showed some signs of misfit to the Rasch model for all three scales (overall item-trait interaction, P ≥ 0.003; reliability, ≥ 0.85). For example, all scales exhibited signs of dependency between item responses, and the PCS measured with relatively low precision. Model fit (but not measurement precision) was improved following deletion of one PCS and one MCS item (overall item-trait interaction, P ≥ 0.387; reliability, ≥ 0.82). These observations suggest that the SF-12 can be used as a coarse health survey tool in PD and that a total SF-12 may be useful as a measure of overall health. However, its appropriateness as an outcome measure can be questioned and it is somewhat unclear exactly what the derived scores represent. As such, the SF-12 should probably be considered an assessment tool (or index) rather than a measurement instrument.