Pain measurement: Visual Analogue Scale (VAS) and Verbal Rating Scale (VRS) in clinical trials with OTC analgesics in headache

Aim: The aim was to assess the performance of the Visual Analogue Scale (VAS) in patients recruited in a clinical trial with over the counter analgesics in headache. Methods: The Thomapyrin Study showed the significant superiority of the fixed combination of acetylsalicylic acid + paracetamol + caffeine over the combination without caffeine, the single preparations, and placebo in the treatment of headache. Patients enrolled into the study were trained in the handling of the VAS by naming categories of a 6-point Verbal Rating Scale (VRS). These data were used to evaluate the level of order consistency between the VAS and VRS, to deduce cut-off points for rescaling the continuous VAS into a discrete ordinal scale using the receiver operating characteristic methodology, and to assess the test-retest performance. Results: Approximately 75% of the patients recorded the pain intensity on the VAS in the same order as given on the VRS. However, in 12.6% of patients, the German terms 'leicht' (mild) and 'massig' (moderate) were mixed up regarding their order on the VAS. Substantial overlapping of the frequency distributions of the VAS assessment were found for the VRS categories mild and moderate pain as well as severe and very severe pain. Grouping of the VAS assessments into a discrete ordinal scale necessitated a non-equidistant rescaling based on the categories of the VRS. By means of analysis of the receiver operating characteristic curves, the following cut-off points were determined on a 100 mm VAS: no pain 0-2 mm, mild pain 2-17 mm, moderate pain 17-47 mm, severe pain 47-77 mm, very severe pain 77-96 mm, most severe pain imaginable 96-100 mm. Repeated assessment up to several months after the first assessment demonstrated a test-retest agreement on the VAS in 61.0-91.4% of the patients, depending on the VRS category. Conclusions: This study shows that the VRS categories cannot be presented in an equidistant manner on the VAS, and that contrary to previous assumptions, the pain intensity descriptors are less clear and can have different meanings in different languages. Therefore, both in the 3rd edition of the International Headache Classification (ICHD-III) and in the guidelines for clinical trials of patients with headache illnesses, rather than a 4-grade VRS, a 6-grade or higher level VRS or a VAS should be recommended, with correspondingly broadly defined anchor points.