OBJECTIVES:

To define the minimum clinically important difference (MCID) for the visual analog scale (VAS) of pain severity by measuring the change in VAS associated with adequate pain control.

METHODS:

The authors conducted a prospective, observational study. Adult emergency department (ED) patients with acute pain (<72 hours) were eligible. Patients rated their pain severity on a 100-mm VAS on presentation and at discharge. Patients were asked if they would accept any analgesic, then if they would accept a parenteral analgesic before treatment. At discharge, they were asked whether they had received adequate pain control.

RESULTS:

The authors enrolled 143 patients (mean age, 36 years; 54% female). The mean decrease in VAS was -30.0 mm (95% confidence interval [CI] = -36.4 to -23.6) for the 116 of 143 (81%) patients with adequate pain control at discharge vs. -5.7 (95% CI = -11.2 to -0.3) for the 27 with inadequate pain control (p < 0.001). At discharge, the mean VAS was 31.3 mm for those with adequate pain control vs. 55.1 for those without. Mean VAS for the 114 of 143 patients who would accept any analgesics initially was 64.7 vs. 47.1 for the 29 reporting no analgesic need. Initially, 77 patients would accept parenteral analgesics (mean VAS = 72.5 mm).

CONCLUSIONS:

A mean reduction in VAS of 30.0 mm represents a clinically important difference in pain severity that corresponds to patients' perception of adequate pain control. Defining MCID based on adequate analgesic control rather than minimal detectable change may be more appropriate for future analgesic trials, when effective treatments for acute pain exist.