Summary of findings and comparison to a previous similar study
This is the second study in which SMS-Track data for non-episodes were studied for a specific study population over one year. The first study revealed non-episodes to be rare in patients with LBP who had been referred to a specialized back hospital [
3]. According to the results in this second study, however, people in the general population enjoy long periods of non-episodes and about 1/5th do not appear to have experienced any LBP at all. Our results therefore indicate that this definition of non-episodes is potentially useful, as its prevalence is logically different in these two very different study populations. In addition, we found that there were a fairly large number of people with at least two non-episodes, indicating that there would be some episodes in this population, not only people either with or without LBP.
In LBP research, using ordinary questionnaire data, it is customary to establish LBP status at the end of the study period, such as at a 6-month or a 12-month follow-up. If ‘recovery’ were studied after twelve months in patients with LBP attending the secondary care sector, it is likely that only a small minority would be considered to be well [
3] whereas in the general population, not surprisingly, somewhat more than half would be considered LBP-free.
Strengths and limitations of the study
A strength of this study is that it deals with a randomly selected sample from the general population. Another strength is that the study participants were all of the same age, which means that there would be no modifying effect of age. This narrow age bracket means, though, that results may not be transferable to those much younger or older. The frequent data collection and the medium through which it was conducted seemed acceptable to the participants, as there were only 16 people who produced insufficient data for inclusion in the study and among the participants only 2% of all cells were missing.
Although the response rate was apparently high, 277 out of 293 (94%), the final study sample did, in fact, only represent 277/412 (67%) of the baseline study sample and just 277/625 (44%) of those initially targeted. This probably results in a highly selected study sample, but if and how this could have affected our results is not known. This declining participation rate in studies with multiple follow-ups is a known phenomenon but it is often ignored in studies with several follow-ups, as response rates are commonly calculated on the basis of those invited in the immediately preceding survey [
10]. Our response rate therefore does not compare unfavourably to those of many other similar studies. However, the purpose of this study was not to establish exact prevalence estimates of episodes and non-episodes but rather to investigate the usefulness of the definition of ‘non-episode’. Other age groups or sub-samples of the general population could, of course, have different profiles.
Another positive aspect of the current study is that the frequent data collection and short reference period is likely to minimise recall inaccuracies. Also, as the data reporting was very consistent (those without pain kept on reporting no pain and those with pain reported pain in a similar manner throughout the whole study period), it is unlikely that the imputed values (only 2% of all cells which were spread over several people) would have falsified the final results.
Nevertheless, when comparing non-episode data between studies, it would be important to take into account the recall period. In this study, we used two weeks. This meant that a non-episode would depend on the absence of LBP over two consecutive fortnights (i.e. one month). However, if one pain-free fortnight was found next to one pain-free week on each side, this would not be identified, if each of those neighbouring fortnights contained at least one day with LBP, as it cannot be seen, in which of the two weeks this day is located. If data had been collected weekly, more pain-free months could therefore have been identified. It is therefore possible that there is some under-reporting of pain-free months in the current study. The choice to collect data fortnightly only was made in order not to fatigue our participants with too many text-messages over the study period. We assumed that it would be easier to obtain compliance 26 rather than 52 times over one year, particularly as this study was conducted on the general population.
Although this study was able to provide unique course data over one year, no information is available on the fluctuation in symptom severity. This would have necessitated more complex questions and answers which were unsuitable for this type of data collection. However, when studying the absence of LBP, symptom severity does not appear important. If a person reports having pain, regardless of its intensity, it is likely that classifying them as not being absent of pain is the correct choice.