Achieving reliable pain change scores for individuals in postoperative phase: carefully choose sampling density, test length, and administration mode.

Despite tremendous efforts to increase the reliability of pain measures and other self-report instruments, improving or even evaluating the reliability of change scores has been largely neglected. In this study, we investigate the ability of two instruments from the Patient-Reported Outcomes Measurement Information System (PROMIS®), pain interference (6 items) and pain behavior (7 items), to reliably detect individual changes in pain during the postsurgical period of a hernia repair in 98 patients who answered daily diaries over almost 3 weeks following surgery. To identify the most efficient strategy for obtaining sufficiently reliable estimates of change (reliability >.9), the number of measurement occasions over the study period (sampling density), the number of items (test length), and the mode of administration (i.e., static short-form vs.computer-adaptive testing [CAT]) were manipulated in post-hoc simulations. Reliabilities for different strategies were estimated by comparing the observed change to the best approximation of “real” (i.e., latent) change. We found 1) that near perfect reliability can be achieved if measures from all days over the whole study period, obtained with all pain interference or pain behavior items, were used to estimate the observed change, 2) that various combinations of the number of items and the number of measurement occasions could achieve acceptable reliability, and, 3) that CATs were superior to short-forms in achieving sufficient reliability. We conclude that the specific strategy for assessing individual postoperative change in pain experience must be selected carefully.
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