Background: Evidence suggests that the course of low back pain (LBP) symptoms in randomised clinical trials
(RCTs) follows a pattern of large improvement regardless of the type of treatment. A similar pattern was
independently observed in observational studies. However, there is an assumption that the clinical course of
symptoms is particularly influenced in RCTs by mere participation in the trials. To test this assumption, the aim of
our study was to compare the course of LBP in RCTs and observational studies.

Methods: Source of studies CENTRAL database for RCTs and MEDLINE, CINAHL, EMBASE and hand search of
systematic reviews for cohort studies. Studies include individuals aged 18 or over, and concern non-specific LBP.
Trials had to concern primary care treatments. Data were extracted on pain intensity. Meta-regression analysis was
used to compare the pooled within-group change in pain in RCTs with that in cohort studies calculated as the
standardised mean change (SMC).

Results: 70 RCTs and 19 cohort studies were included, out of 1134 and 653 identified respectively. LBP symptoms
followed a similar course in RCTs and cohort studies: a rapid improvement in the first 6 weeks followed by a
smaller further improvement until 52 weeks. There was no statistically significant difference in pooled SMC between
RCTs and cohort studies at any time point:- 6 weeks: RCTs: SMC 1.0 (95% CI 0.9 to 1.0) and cohorts 1.2 (0.7to 1.7);
13 weeks: RCTs 1.2 (1.1 to 1.3) and cohorts 1.0 (0.8 to 1.3); 27 weeks: RCTs 1.1 (1.0 to 1.2) and cohorts 1.2 (0.8 to 1.7);
52 weeks: RCTs 0.9 (0.8 to 1.0) and cohorts 1.1 (0.8 to 1.6).

Conclusions: The clinical course of LBP symptoms followed a pattern that was similar in RCTs and cohort observational
studies. In addition to a shared ‘natural history’, enrolment of LBP patients in clinical studies is likely to provoke responses
that reflect the nonspecific effects of seeking and receiving care, independent of the study design.

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