Pain intensity assessment scales are important in evaluating analgesic choices and therapeutic effects. The 100-mm visual analog scale (VAS), a unidimensional scale that is versatile and easy to use, has been adopted in many settings. A major study of trauma patients concluded that a 13-mm difference on the VAS represented the smallest measurable change in pain severity that is clinically important and identifies patients having “a little more pain” or “a little less pain.” Gallagher and associates performed a prospective, observational cohort study in two urban emergency departments to test the hypothesis that 13 mm is the minimum clinically significant difference in acute pain intensity on the VAS scale.
Ninety-six patients with acute pain were asked to make an initial vertical mark on the pain scale and then repeat the scoring on a new VAS every 30 minutes for two hours. After completing each VAS measurement, patients were asked to contrast their current pain with the pain at the time of the previous measurement, using one of the following five descriptors: (1) much less pain, (2) a little less pain, (3) about the same pain, (4) a little more pain, or (5) much more pain. Patients who described their pain as “about the same” had a median and mean change in VAS score of zero mm. The distribution of changes in VAS scores for ratings with “a little more pain” or “a little less pain” was wide, but the mean change was 13 mm.
The authors conclude that detection of the minimum clinically significant difference in pain, as identified by patients noting a little more pain or a little less pain, is approximately a 13-mm change in the VAS linear score. The wide variation among patients, however, remains a limitation in using the VAS as a research tool to evaluate analgesic treatments.
In a related article, Bird and Dickson studied the VAS pain scale change as a clinically significant change in pain. They concluded that when the initial VAS score was less than 34, a 13-mm change represents a clinically significant change in pain; however, among patients with initial VAS scores of at least 67, a 28-mm change was needed to represent a clinically significant change in pain. This hints that patients having more pain need a greater change in VAS score to demonstrate clinically significant lessening of pain.