This systematic review reveals that there are relatively few studies which have investigated the diagnostic accuracy of tests to identify the disc, facet joint or SIJ as the source of low back pain. Only two index tests (MRI-HIZ and MRI- disc degeneration) have been investigated by five or more studies. Only a few studies evaluated a cluster of signs or a combination of tests. The results of the SIJ studies found increased diagnostic validity for a cluster of tests compared to a single test in isolation. Forming a diagnosis based on a combination of findings is typical of the clinical reasoning approach used by clinicians and should be investigated in future studies.
The results of studies investigating the disc as the source of low back pain indicate that there is no available clinical test which can be used to both increase and to decrease the likelihood of the disc as the source of low back pain. However, several of the available tests (MRI high intensity zone, MRI disc degeneration, MRI endplate changes, and centralisation) have informative +LRs indicating that a positive test result does increase the likelihood of the disc as the source of the patient’s symptoms. The results however are heterogeneous making an accurate prediction of diagnostic strength impossible. Reduced MRI signal intensity is the only index test, which decreased the likelihood of the disc as the source of symptoms and then only when a low threshold is used. When the lowest threshold available in the eight studies was used, all studies found informative −LRs. The data approached statistical heterogeneity (P = 0.03) and a pooled estimate for −LR was 0.21 (0.12–0.35) demonstrating moderate ability for a negative MRI to rule out the individual disc as a source of symptoms.
The results of studies investigating the facet joint as the source of a patient’s symptoms suggest that the currently available tests have limited or no diagnostic validity. Studies of “Revel’s criteria” found conflicting results. However, the only study that used a double block found no useful diagnostic value. Two clinical prediction rules developed by Laslett [27
] (Appendix 2) have both informative +LRs and –LRs. However, these have only been developed in a single study and need validating in an independent sample.
A combination of SIJ pain provocation tests appears to be useful both to increase and to decrease the likelihood of the SIJ as the source of symptoms in patients with pain primarily below the fifth lumbar vertebrae. The summary +LR and −LR of 3.19 and 0.29, respectively suggest moderate changes to the post test probability. While a positive bone scan appears to be useful at increasing the probability of the SIJ being the source of low back pain, it also has very low sensitivity, which means that the majority of patients with pain from the SIJ will have a negative bone scan.
The tests reviewed produce small or at best moderate changes in pre to post-test probability. For example assuming a pre-test probability of 50% for the disc being the source of pain a +LR of 3, as was typical for high intensity zone and centralisation studies, would change the post test likelihood to 75%. The −LR of 0.21 for absence of disc degeneration would reduce the likelihood of the disc as the source of pain to 17%. Assuming a lower pre-test probability of 20% for the SIJ as the source of pain the +LR of 3.19 for the combination of SIJ tests would increase the likelihood to 45%. The −LR of 0.29 for the SIJ tests would reduce the probability to 7%. These changes in probability of the disorder are modest but must be considered in the context of current recommendations that it is impossible to identify a source for a patient’s low back pain.
The results of this study may be used in future research to identify patients more likely to have pain originating from the disc or SIJ and test the effectiveness of treatments aimed at these structures. Currently there is no literature indicating that knowledge of the tissue source of low back pain leads to improved outcomes however this research has been very difficult to perform without easily available and valid methods of identifying the source of low back pain.
The results of this study rely on the accuracy of the reference standards used. There has been much controversy in the literature on discography [8
] and to a lesser extent facet and SIJ blocks.[14
] A high rate of false positive responses to discography and facet blocks has been reported in the literature by some authors [16
]. Other authors have found low false positive rates especially when strict criteria for a positive response are used [10
]. In our review we required relatively strict criteria for a positive response to discography (concordant pain and a minimum of two levels tested per patient) and to facet and SIJ injections (at least 50% pain reduction with guided injection). We pre-planned to investigate the impact of even stricter reference standards including a pain free adjacent disc or positive morphology, for discography and higher levels of pain relief or a pain free control injection for facet joint or SIJ blocks. However, there were not enough studies using the higher level of control to investigate if this impacted on the diagnostic validity of different index tests.
One of the limitations of the studies included in our review was that the majority of patients in the trials may not be representative of patients presenting for care of their low back pain. The patients were primarily a convenient sample of patients presenting for each type of diagnostic injection and may be more likely to have the target condition than an unscreened cohort presenting for care of low back pain. There is a need for research to be done in less selected populations however these studies may be difficult to conduct due to the invasive nature of the reference tests. The prevalence of the target disorder varied considerably across the included studies. This implies the populations were dissimilar and some pre-selection bias may have occurred. This may be a primary cause of heterogeneous results making pooling impossible.