Defining chronic low back pain: Should we include the number of pain days?

Chronic low back pain (CLBP) is typically defined in terms of pain duration (>3 months) only. However, categorising patients with occasional and persistent LBP together increases clinical heterogeneity in study populations. To address this, the NIH Research Task Force on CLBP recommended including the number of pain days in the CLBP definition (i.e., LBP on more than half of the days over the past 6 months), but its implications remain unclear. In this cross-sectional study, 353 adults with CLBP (>3 months; LBP ≥1 day/week) completed an online survey assessing sociodemographic and clinical characteristics, including the average number of LBP days/week. The number of pain days/week was moderately associated with pain intensity and disability, but weakly associated with psychological and sociodemographic factors. Participants reporting LBP 4-6 days/week did not differ from those with daily pain, whereas those experiencing LBP 1-3 days/week reported significantly lower pain intensity and disability. Importantly, 50.8-67.2% of participants with 2 or 3 pain days/week still experienced clinically relevant pain intensity or disability. When dichotomised, participants with LBP on ≥4 days/week were more often women and reported higher pain intensity, disability, and symptoms of anxiety and depression than those with <4 days. Finally, psychological factors were more strongly associated with disability in the ≥4-day group compared to the <4-day group. In conclusion, while ≥4 days/week may represent a clinically meaningful cut-off, this criterion excludes a substantial subgroup with clinically relevant CLBP, for whom findings in populations with more frequent LBP may not be representative. PERSPECTIVE: Pain frequency reflects meaningful differences in pain intensity and disability in CLBP. While higher-frequency pain identifies a more severe clinical subgroup of patients, individuals with fewer pain days can still experience substantial impairment, underscoring the need for nuanced CLBP definitions in research and clinical practice.