A limitation of this study is that about one-third of patients
developing persistent LBP were not correctly identified in
the final four-predictor model. Also, that we did control for
age, gender and body mass index but not for smoking in order
not to lose too many cases due to the many missings for the
variable smoking. Furthermore, the predominant use of
patient-reported outcome measures for generating information
is subjective by nature. Finally, attrition bias can be seen
as a threat to the representativeness of the study sample.
However, a recent study found that attrition has only
marginal influence on the point estimates of LBP-related
outcomes (Schmidt et al. 2011). In the present study, the loss-to-follow-up was consistently about 15 % at each follow-
up time point. This means that this loss was a systematic
one and not due to any specific event. The total loss-tofollow-
up was 46 % over the whole study period. This
apparently high rate should be considered in the context of a
postal survey, where direct contact with the participants was
limited to the initial screening interview. A recent study on
342 LBP patients presenting in primary care was followed up
six times over a 6-month period and showed a comparable
loss-to-follow-up of 45 % (Dunn et al. 2006).