1 Department of Rehabilitation, Faculty of Medicine, Université Laval & Centre for Interdisciplinary Research in Rehabilitation and Social Integration, Quebec City, Canada.
2 Department of Health Sciences, Université du Québec en Abitibi-Témiscamingue (UQAT), Rouyn-Noranda, Canada.
3 Centre de Recherche du Centre Hospitalier de l'Université de Montréal (CRCHUM), Université de Montréal, Montreal, Canada.
4 Centre de Recherche du Centre Hospitalier Universitaire de Sherbrooke (CHUS)-Department of Anesthesiology, Université de Sherbrooke, Sherbrooke, Canada.
5 Department of Health, Kinesiology & Applied Physiology, Concordia University, Montreal, Canada.
6 Research Center on Aging, CIUSSS de l'Estrie-CHUS, School of Rehabilitation, Université de Sherbrooke, Sherbrooke, Canada.
7 Department of Anesthesiology, University of Minnesota, Minneapolis, MN.

Introduction: Numerous definitions of acute low back pain (aLBP) exist. The use of different definitions results in variability in reported prevalence or incidence, conflicting data regarding factors associated with the transition to chronic LBP (cLBP), and hampers comparability among studies.

Objective: Here, we compare the impact of 3 aLBP definitions on the number of aLBP cases and participants' characteristics and explore the distribution of participants across definitions.

Methods: A sample of 1264 participants from the Quebec Low Back Pain Study was included. Three definitions of aLBP were used: (1) not meeting the National Institutes of Health (NIH) cLBP definition ("nonchronic"), (2) pain beginning <3 months ago ("acute"), and (3) pain beginning <3 months with a preceding LBP-free period ("new episode").

Results: There were 847, 842, and 489 aLBP cases meeting the criteria for the 3 definitions, respectively. Participants included in the "nonchronic" had lower pain interference, greater physical function scores, and fewer participants reporting >5 years of pain than in the other definitions. Half the participants meeting the "acute" definition and one-third of participants meeting the "new episode" definition were also classified as cLBP based on the NIH definition.

Conclusions: Our results highlight the importance of the definition used for aLBP. Different definitions influence the sample size and clinical profiles (group's characteristics). We recommended that cohort studies examining the transition from aLBP to cLBP ensure that the definitions selected are mutually exclusive (ie, participants included [aLBP] differ from the expected outcome [cLBP]).