Is the pain VAS linear and responsive to change?
The Pain Visual Analogue Scale (VAS) is often used by clinicians and in clinical trials as a primary or secondary outcome tool. Patients are asked to rate the severity of their pain on the 10cm marked scale at one point in time or after different intervals.
This paper examines the properties, validity and responsiveness of the pain VAS by using Rash analysis. This analysis examines if people use the scale as expected.Patients often find it difficult to rate their pain and the authors argue that this could be due to the fact that patients do not score it as an interval scale.
The analysis showed that the VAS in internally valid and is a valid tool for measuring pain at one point in time. However, there is a lack of sensitivity to distinguish between groups of people with different levels of pain and this often results in either a under- or overestimate of true change. It challenges the interpretation of pain VAS change scores as reported in the literature and advocates for additional statistical methods prior to data interpretation.