OUTCOME MEASURES AND POWER IN NON-SPECIFIC LOW BACK PAIN TRIALS

Abstract

Randomised controlled trials (RCTs) of non-specific low back pain (LBP) interventions commonly estimate sizes of treatment effect from subjective scales. It is not clear which scales are most commonly used. Moreover, the standardized effect sizes, d, of interventions are typically small (d ≈0.3) and many trials may be underpowered to detect d at this magnitude, regardless of scale used. We identified RCTs reported between 1980 and 2000 from the European guidelines for low back and extended the search to 2006 using MEDLINE, EMBASE, PsycINFO, Cochrane, and Lilacs. We extracted data from identified studies concerning outcome measure use and power to detect d. Following analysis of 222 papers, the most commonly used outcome measure was the visual analogue pain scale (VAS-P), used by 42% of trials, followed by the Roland Morris disability questionnaire (RMDQ), used by 34% of trials. Secondary measure use was diverse; 5% of trials included 10 or more measures and 40% involved at least one objective measure. 57% of the trials were powered to detect d = 0.8, only 37% could detect d = 0.5, only 6% were able to detect d = 0.3 and only one trial could detect Cohen’s definition of a small effect, d = 0.2. In sum, the VAS-P is the most commonly used outcome measure in trials of non-specific LBP and the majority of trials are too small to show effect sizes that might realistically occur.