The main objective of this study was to examine the ability of three commonly performed clinical tests to discriminate between CLBP patients and healthy controls. Our results revealed discriminative ability for the MCI test, but not for the BPA and TPD tests. Consequently, we can only recommend MCI as a test for this discrimination, due to its statistically significant odds ratio of .65 for the likelihood of having LBP.
This finding for MCI confirms previous results, in which discriminative ability was found between LBP patients and healthy controls (12, 18–21). Luomajoki et al. revealed a mean MCI score (out of 6) of 2.21 in patients with LBP and 0.75 in healthy controls (21). In contrast, both of our groups had higher MCI scores: 3.0 for CLBP patients and 2.0 for healthy controls. However, this current study only included patients with CLBP, whereas Luomajoki et al. also investigated patients with acute and subacute LBP (21).
Our results for the BPA test differ from previous research. Moseley et al (2005) used a similar approach with CRPS patients, demonstrating that patients selected photos with a 7% enlargement of the original size in the chronic pain group (22). In our study, both CLBP patients and healthy controls tended to choose enlarged photos of their backs and showed no meaningful between-group difference. Our BPA results indicate no significant association between an increased BPA score and a higher chance of having CLBP. However, different enlargement steps used in the Moseley et al. study could partly explain the different results. Our photos were modified in 3% steps, whereas Moseley et al. used 5% steps. In contrast to our study, the latter also demonstrated a correlation between the chosen picture and the duration of symptoms (22). BPA is a rather novel test for the detection of altered body perception, based on only preliminary data for CRPS patients, but it has not yet been validated for CLBP patients. Furthermore, our study investigated the trunk, whereas Moseley et al. focused on limbs, showing pictures of their both hands to participants. In the Moseley et al. study, the affected hand was modified, thereby allowing a comparison of both hands. In contrast, we only showed modified photos of one area at level, L4. Hence, the results of the two studies cannot be compared directly.
Our findings for the TPD test also diverge from previous studies that demonstrated a discriminative ability of TPD between CLBP patients and healthy controls (15, 17, 37). Luomajoki and Moseley also observed a correlation between the TPD and MCI tests (17). However, our study found no statistically significant correlation between TPD and MCI. Furthermore, no statistically significant correlations among the independent variables were found. This accords with previous research. Ehrenbrusthoff et al. questioned the correlation between TPD and the FreBaQ questionnaire, by doubting the similarity of the underlying construct (38). However, recent studies have demonstrated correlations between TPD and body image drawings, which is another visual approach similar to BPA (23, 39).
This leads to the question of whether low baseline pain intensity of 33.4/100 VAS and disability of 8.2/24 in our patient cohort, which may cause less alteration at the cortical level could explain the inability of the included tests to demonstrate significant discriminative capability. Nevertheless, it remains unclear as to whether higher pain intensities and disability levels would result in greater discriminative ability between CLBP and healthy controls.
A methodological limitation of this study was that two different assessors performed the assessment of TPD to maintain blinding of all assessors. Catley et al. (2013) questioned the inter-rater reliability of TPD at the lower back (24). Therefore, this could have influenced our results.
Additional research is needed to improve BPA.. We suggest that enlargement steps should be increased from 3–5% and that the impact of smaller increments on significance be investigated. It is also unclear whether CLBP patients with higher activity levels would have different outcomes to CLBP patients with lower activity levels. Research has shown a change of proprioception and, therefore, movement control due to exercise (1, 40, 41). Moreover, a screening of the sample according to the criteria of the avoidance-endurance model of pain may result in more consistent findings.