External Validation of the Minimum Clinically Important Difference in the Timed-up-and-go Test After Surgery for Lumbar Degenerative Disc Disease

Study Design. Prospective observational cohort study. Objective. The aim of this study was to provide external validation of the minimum clinically important difference (MCID) of the Timed-up-and-go (TUG) test. Summary of Background Data. The TUG test is one of the best explored and most frequently applied objective task-based functional outcome measure in patients with lumbar degenerative disc disease (DDD). The increased use of the TUG test is based on its solid psychometric properties; however, an external validation of the originally determined MCID is lacking. Methods. Forty-nine patients with lumbar DDD, scheduled for elective spine surgery, were assessed pre- and 6-weeks (W6) postoperative. MCID values were calculate for raw TUG test times (seconds) and standardized TUG z scores using three different computation methods and the following established patient-reported outcome measures (PROMs) as anchors: Visual Analog Scales (VAS), Core Outcome Measures Index Back, Zurich Claudication Questionnaire (ZCQ). Results. The three computation methods generated a range of MCID values, depending on the PROM used as anchor, from 0.9 s (z score of 0.3) based on the VAS leg pain to 3.0 seconds (z score of 2.7) based on the ZCQ physical function scale. The average MCID of the TUG test across all anchors and computation methods was 2.1 s (z score of 1.5). According to the average MCID of raw TUG test values or TUG z scores, 41% and 43% of patients classified as W6 responders to surgery, respectively. Conclusion. This study confirms the ordinally reported TUG MCID values in patients undergoing surgery for lumbar. A TUG test time change of 2.1 seconds (or TUG z score change of 1.5) indicates an objective and clinically meaningful change in functional status. This report facilitates the interpretation of TUG test results in clinical routine as well as in research.