Background: People with idiopathic Parkinson's disease (PD) develop postural instability in the later stages of the ailment. Postural instability has traditionally been quantified with the Pull test even though its face validity is limited. We previously established cut-off scores for a three-part rapid assessment of postural instability (RAPID) questionnaire as a non-physical complement to the physical test. In the current study, the questionnaire was administered to a new group of PD subjects to evaluate the diagnostic value of the instrument. Methods: Sensitivity and specificity values were calculated for single and combined sections of the questionnaire by using the Pull test as the gold standard for assessing the presence of postural instability. Results: The questionnaire when used in its entirety gave the highest sensitivity (.71), whereas specificity was highest in the activities of daily living (.74) and fear of falling sections (.74). Net specificity decreased to.44 when the scores from the three sections of the questionnaire were combined. Conclusions: The high sensitivity of the RAPID questionnaire suggests that it may be used as an adjunct to the Pull test or solely if it is not convenient or contraindicated. The questionnaire may also be adapted for use via the telephone or internet. The limitation of the Pull test in revealing postural instability may explain the low specificity of the questionnaire, i.e. the questionnaire correctly identifies patients as unstable when the Pull test indicates normal postural control. It is hoped that the rapid identification of postural instability in PD may lead to increased awareness of the disease progression and fewer falls.
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