Reliability and validity of a visual analog scale for acute abdominal pain in the ED

The objective of the study was to assess the validity and reliability of the visual analog scale (VAS) in the measurement of acute abdominal pain, and to identify the minimum clinically significant difference in VAS scores among patients with acute abdominal pain. The study was undertaken in preparation for a randomized clinical trial of opioid use in acute abdominal pain. A prospective, observational cohort study of a convenience sample of patients presenting to 2 urban EDs with the chief complaint of acute abdominal pain was conducted. At time 0 and 1 minute later each subject indicated pain severity on a 100mm VAS. This was repeated every 30 minutes for 2 hours. Patients were also asked to contrast their current pain severity with their pain in the preceding 30 minutes using one of 5 graded verbal descriptors: "much less pain," "little less pain," "the same pain," "little more pain," and "much more pain." Validity was assessed by performing an analysis of variance for linear trend on the association between the 5 categorical pain descriptors and change in VAS scores. Reliability was assessed using the intra-class correlation coefficient (ICC) between VAS scores taken 1 minute apart, supplemented by a Bland-Altman analysis. The minimum clinically significant difference in pain was defined as the mean difference between sequential VAS scores obtained 30 minutes apart when the patient noted a "little less" or "little more" pain. Differences in VAS scores increased linearly as pain descriptors escalated from "much less" to "much more" pain (P <.001). Reliability was high, ICC = 0.99 [95%CI 0.989 to 0.992] for 0 and 1 minute VAS scores. The minimum clinically significant difference in acute abdominal pain was 16 mm (95% CI 13, 18 mm). VAS measures of acute abdominal pain are valid and reliable. The 95% CI surrounding the minimum clinically significant difference of approximately 16 mm overlaps with the 95% CI of minimum clinically significant difference of approximately 13 mm reported previously in traumatic and other types of acute pain. We conclude that the VAS is a methodologically sound instrument for quantitative assessment of acute abdominal pain and for detecting clinically important changes in such pain.