The Functional Pain Scale: Reliability, Validity, and Responsiveness in an Elderly Population

Objectives: Because of difficulty experienced in assessing pain in frail older patients and the lack of pain assessment tools with standardization in the elderly, the Functional Pain Scale (FPS), an instrument incorporating both subjective and objective components to assess pain, was developed and evaluated.

Design, Setting, Participants, And Measures: One hundred subjects more than 65 years old participated in the validity, reliability, and responsiveness (the clinical sensitivity of the instrument to change) testing of the Functional Pain Scale. Subjects were recruited from a geriatrics inpatient setting, a geriatrics outpatient setting, and a local hospice (residing in their homes). Ninety-four of the subjects completed all phases of testing. Reliability was tested using a test-retest format and a correlation matrix. Criterion-related validity was established as compared with the Visual Analog Scale (VAS), the Present Pain Intensity (PPI), the McGill Short Form Questionnaire (MPQ-SF), and the Numeric Pain Scale (NPS) instruments. Responsiveness for the FPS, the VAS, the PPI, the MPQ-SF, and the NPS instruments was determined using five previously described techniques: effect size, standardized response means, relative efficiency, direct comparison of t test scores, and direct comparison of P values. A cumulative index was developed to rank each scale. Cumulative responsiveness index scores were based on individual scale performance for each separate responsiveness test. The lowest score in the cumulative responsiveness index indicated the most responsive scale.

Results: Interrater reliability for instruments tested exceeded 0.95 for all instruments tested. Validity testing showed high correlations as well (r = 0.62, r = 0.85, r = 0.80, r = 0.90 for the VAS, the PPI, the MPQ-SF, and the NPS respectively). Responsiveness evaluated overall by the responsiveness index was best for the Functional Pain Scale (7) followed by the Visual Analog Scale (12), the Present Pain Intensity (13), the McGill Pain Questionnaire-Short Form (19), and the Numerical Pain Questionnaire (24).

Conclusions: The Functional Pain Scale was determined to be reliable, valid, and responsive. The responsiveness of the Functional Pain Scale was superior to the other instruments tested. The Functional Pain Scale is an acceptable instrument for assessing pain in older adults and may reflect changes in pain better than other instruments tested. Further testing in other populations is warranted.