Assessing smallest detectable change over time in continuous structural outcome measures: Application to radiological change in knee osteoarthritis

Interpreting changes in continuous structural outcome measures is a common problem in clinical research and in daily practice. We propose a method for estimating whether difference observed between two successive measures in an individual constitutes a statistically relevant change or a change induc ed by variability. This statistically relevant change is based on an analys is of reproducibility. The continuous structural outcome measure investigat ed as an example was joint space width (JSW) measurement on standard X-rays , which is known to be the primary end-point for assessing structural osteo arthritis progression. The results of the present study demonstrate that cu toffs are closely dependent on all sources of variabilities in JSW measurem ent such as joint positioning, radiographic procedure, and the measurement process itself. Therefore, we suggest to determine cutoffs for each study u sing a representative sample of the population studied and using the proced ures and methods of measurement of the specific study. This approach may ea sily be extended to other continuous structural outcome measures. (C) 1999 Elsevier Science Inc.