This appears to be the first study to compare baseline characteristics of LBP patients to pain patterns generated by very frequent follow-ups over a period of time. Moreover, it was the first attempt to study whether the prognosis of primary care patients with LBP is related to diagnostic classes as defined by the classification system described by Petersen [10
]. Although we had a relatively small study sample, and a large number of subgroups, it was still possible to obtain some useful information.
First, it appears that at least some of the diagnostic classes relate to the prognosis. Patients classified as having disc-related pain reported more pain days and were less likely to experience the pain course 'mainly recovered' than others. Patients with disc pain had on average between 13 and 19 more days with pain than patients with muscle pain, mechanical dysfunctions, or SI-joint pain. It would be relevant to investigate such differences in more depth including whether diagnostic classes differ not only regarding pain, but also in relation to activity of daily living or disability. If similar associations between diagnosis and prognosis are confirmed by other studies, the differences are large enough to be important to patients and indicate that this classification system makes a distinction between relevant subgroups of patients.
In accordance with previous studies [26
] men had a better prognosis than women. They had fewer days with LBP in total, were more likely to undergo the course pattern 'mainly recovered', and seemed to have less fluctuating patterns than women. The present results suggest that this could be, at least partly, explained by the difference in diagnostic classes between men and women, since men were less often classified with disc pain than women were. In addition, age was related to outcome patterns in the way that young patients had a milder course than older. The present cohort was not large enough to explore in more detail whether certain pain patterns relate to each gender or certain age groups and this should be explored in larger studies. In accordance with previous cohort studies on chiropractor patients [15
], but maybe surprising to many clinicians, the duration of the present LBP episode was not associated to any of the outcome measures.
Because of the small numbers within each diagnostic class, statistical testing in relation to agreement was unworkable and the agreement was therefore only evaluated in percentages that do not take into account agreement by chance. The agreement concerning the diagnostic classes was high when based on the most manifest class, and markedly lower if absolute agreement was demanded. However, we consider the obtained agreement sufficient for the classification to be meaningful. The reliability of the classification system was tested in a set up with two chiropractors being present at the same consultation. This could have introduced bias toward higher agreement, but was chosen to avoid an altered symptom response at the second examination. The same decision was made in earlier studies [25
]. The agreement on all classes was high (54%) as compared to a previous study on this classification system [25
] with 34% agreement. The main difference, between the methods of the previous study and our, was that we allowed the use of more than one of the diagnostic classes. In the original study the classes were described as mutually exclusive. Therefore, in our study, it was possible to use more classes instead of making a compulsory final choice between two seemingly relevant classes. This approach seems reasonable because pain can be generated from more than one structure. We are aware of studies concluding that disc pain very seldom coexists with facet joint or SI-joint pain [28
] and that pain is not likely to originate from both facet- and SI-joints at the same time [29
]. However, these studies included only few patients who were not recruited from primary care, and in our analyses only one class was included in the analyses, consistent with the intention of the classification system.
The main limitation of this pilot study was the relatively large drop out from follow-up. As discussed in previous papers [20
] this was in line with other primary care studies in which patients were followed up less frequently [16
]. Fortunately, baseline characteristics in those who dropped out resembled those of the compliant patients. We suppose that a more enthusiastic information strategy directed to the participating patients could have helped maintaining the interest of the patients.
As a consequence of the quite small cohort we chose to pool the three disc classes from the original classification system into one. This may limit the prognostic value of the classification since we did not distinguish between mechanically reducible and irreducible discs, i.e. pain that can be centralized and pain that cannot, which is known to be of predictive value [30
In conclusion, our results suggest that different diagnostic classes have different pain courses and indicate that patients with different low back conditions can be identified through the physical examination. The next step will be to perform a large-scale practice based study with a sufficient number of patients to make it possible to include more of the diagnostic classes and evaluate prognosis within each of these.