Is the pain visual analogue scale linear and responsive to change? An exploration using Rasch analysis

Area of Science:

• Pain research
• Psychometrics
• Clinical trial methodology

Background:

• Pain visual analogue scales (VAS) are frequently used in clinical trials.
• Their use as interval-level data lacks sufficient evidence.
• This study investigates the validity and responsiveness of pain VAS.

Purpose of the Study:

• To examine the internal construct validity of pain VAS.
• To assess the responsiveness of pain VAS using Rasch analysis.
• To determine if pain VAS data can be treated as interval-level data.

Main Methods:

• Rasch analysis was applied to pain VAS data from 221 patients with chronic joint pain.
• Internal construct validity and fit to the Rasch model were assessed.
• Responsiveness was evaluated using Standardized Response Means (SRM) on both raw and interval data.

Main Results:

• Baseline pain VAS scores generally fit the Rasch model.
• SRMs from ordinal data overestimated true responsiveness by 59%.
• Non-linear behavior was observed, with varying responsiveness across the scale.

Conclusions:

• Pain VAS is valid for single-time point pain assessment.
• Raw pain VAS data should not be used for change-score analysis or power calculations in RCTs.
• Rasch analysis is crucial for converting ordinal VAS data to interval data for accurate interpretation.