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Prospective, longitudinal cohort studyTo examine the validity of a threshold that has been used to define a successful outcome for patients with low back pain (LBP), undergoing nonsurgical rehabilitation based on a 50% improvement on the Modified Oswestry disability index (ODI).Making research findings interpretable is a goal of evidence-based practice. One attempt to improve interpretability is reporting treatment results as the percentage of patients achieving a threshold level of improvement within treatment groups along with mean between-group differences. The most recommended threshold is the minimum clinically important difference of the outcome tool. For clinical conditions with favorable natural histories such as LBP, thresholds requiring more than minimal improvement may be preferable for defining success.Patients with LBP receiving 4 weeks of physical therapy were examined. The ODI and measures of pain, fear-avoidance beliefs, and demographic characteristics were recorded at baseline and after treatment. A 15-point global rating of change was also completed after treatment. The percent ODI change with treatment was computed and compared between groups known to have different prognoses. The percent ODI change was compared to the global rating of change to determine the accuracy of various thresholds of success based on the percent ODI change.A total of 243 subjects (mean age 37.2 ± 11.4 years, 44.9% female) were included. Mean percent ODI change was 43.1% (±40.5), and 109 subjects (44.9%) had a successful outcome (≥50% ODI improvement). As hypothesized, baseline factors with known prognostic importance were less likely to be present in subjects with a successful outcome. The 50% ODI improvement threshold for success had high sensitivity (0.84; 95% CI: 0.79, 0.88) and specificity (0.89; 95% CI: 0.85, 0.93) when compared with success based on the global rating of change. No other percent improvement threshold for the ODI had a higher accuracy than the 50% threshold when compared to the global rating of change.A threshold of 50% improvement on the ODI may be a valid measure for defining a successful outcome for patients with LBP.