Pain and depression often co-exist[1–3], and although the causal relation between the two is not clear[4, 5], evidence suggests that pain negatively affects outcome in depression as well as vice versa[6].

Low back pain (LBP) is a highly frequent pain condition with a substantial impact on global health[7] for which the risk of a poor prognosis is increased in the presence of depression[8, 9]. It is a condition for which there is no generally effective treatment, but non-pharmacological treatment addressing psychological symptoms in addition to the physical symptoms has been demonstrated to improve outcome in LBP patients with high scores on psychological questions[10].

Chiropractors see a large number of LBP patients who appear to be a population with relatively low frequency of severe psychological distress[11–13]. Nevertheless, a substantial proportion with borderline depressive scores has been observed[12]. Therefore, to improve care and to predict prognosis, it may be important to identify psychological factors, including depression, among those seeking care for LBP, also in chiropractic practice. For that purpose screening instruments generally perform better than clinical impressions[14, 15], and a number of questionnaires screening for depression exist[16]. The Major Depression Inventory is thoroughly validated as a diagnostic screening instrument[17, 18] and has been shown to be feasible in chiropractic care[13]. However, it is unknown if the MDI predicts outcome in chiropractic patients.

Despite the potential implications for management and prognosis, routine screening for psychological factors is not widely implemented[19] and has met some resistance from clinicians[20]. One reason might be that the questionnaires are too extensive for routine clinical use where information on a variety of other prognostic factors is also relevant to collect. This is especially true in chiropractic practice where the subpopulation with depressive symptoms is small[11–13], and therefore the identification of these relatively rare cases involves screening of many for whom it has no relevance.

If systematic screening for depression is to be implemented, there is a need for a very short and easily completed tool. Fair to good diagnostic accuracies of one- and two-item screening tools with full questionnaires for depression as reference standards have been demonstrated[21–23]. It is unknown whether observed variations are due to differences between screening questions or between screened populations, and it is possible that suitable screening questions differ between populations. Furthermore, the usefulness of brief screening questions for prediction of prognosis is unrevealed.

The aim of this study was to investigate whether the MDI is predictive of outcome in chiropractic practice and if so, whether one or two items from the MDI could potentially be used as an ultra-short tool for capturing depressive symptoms in LBP patients seeking chiropractic care. To obtain that we tested whether the total MDI was associated with 3- and 12-months outcomes, explored which single items of the MDI correlated best with the total score, and compared the predictive capacity of these items to that of the total MDI.

The study was incorporated in a previously described cohort study with data collection occurring at seventeen chiropractic clinics in the research network of the Nordic Institute for Chiropractic and Clinical Biomechanics in Denmark[24]. Patients completed questionnaires in the reception area prior to the first consultation due to a new episode of LBP. Follow-up questionnaires were mailed after 3 and 12 months. Treatment was unaffected by study participation and the chiropractors were free to choose the treatment they found appropriate. It was confirmed by The Regional Scientific Ethical Committees for Southern Denmark that the study did not need ethics approval according to Danish rules[25].

Our main finding was that item 1 (feeling low in spirits or sad) and item 3 (lacking energy and strength) of the MDI were reasonable alternatives to the total scale when ultra-brief screening questions are needed. In addition we found that depression measured by the MDI was associated with prognosis in patients with LBP, but prediction of outcome had low accuracy.

As in previous studies[11–13] depression was not frequent in this cohort of patients seeking chiropractic care for LBP. Therefore it was not surprising that depression on its own did not predict outcome with any certainty given that a high number of factors appear to influence the course of LBP[8]. One previous study investigating depression as a prognostic factor in chiropractic patients with LBP used the Hospital Anxiety and Depression Scale and found that depression was associated with outcome but not an independent predictor[12]. It was not within the scope of this paper to investigate whether MDI scores were independently associated with outcome. Moreover, it was not investigated if cognitive elements were included in treatment for patients with signs of depression which may reduce the predictive value of depression.

A recent study demonstrated substantial overlap between depression and pain-related psychological factors such as anxiety, self-efficacy, and kinesiophobia, and the combined construct of pain-related emotional distress was an important indicator of risk of poor prognosis in patients from general practice[31]. To examine complex psychological constructs in large clinical cohorts, simple ways of collecting information on each element are essential.

We suggest that items 1 and 3 from the MDI or the two in combination can be used as reasonable substitutes for the MDI scale, although we recognise that items 2 and 4 performed almost correspondingly. With the applied categorisation of MDI, the recommended items predicted activity limitation as well as the total MDI did and were only slightly less accurate in the prediction of pain. The accuracy in terms of AUC and the negative likelihood ratio favoured item 3 slightly over item 1, whereas item 1 should be preferred if positive predictive value is the main priority.

A 2-item screening test for depression taken from the Primary Care Evaluation of Mental Disorders Procedure (PRIME-MD) has previously been recommended for physical therapist treating LBP[15]. The PRIME-MD questions “During the past month, have you often been bothered by feeling down, depressed, or hopeless?” and “During the past month, have you often been bothered by little interest or pleasure in doing things?” reflects the same aspects of depression as MDI items 1 and 2. However, our results indicated that in the investigated setting, item 3 should be preferred to item 2. This was likely due to item 3 having high scores considerably more frequently than item 2 in our cohort. Also 1-item questions simply asking if patients feel depressed[22], or to what extent they have felt depressed during the last month[23], appear useful for diagnostic screening. However, none of these questions were tested as prognostic factors and future research in this area should provide direct comparisons of existing brief screening questions.

This study was based on a large sample from ordinary clinical practice with rather complete data available. Drop-out rates of 21% and 26% at follow-up was a limitation, and slightly higher MDI scores at baseline in those who dropped out may have resulted in underestimation of the effect of depression on outcome. However, there is no reason to believe that single items would be affected differently by drop-outs than the total scale, and we do not consider drop-out a major issue in relation to the aim of this study.

Another limitation was the categorisation of the MDI which reduced the available information but was performed since the continuous measure did not fit a linear model. We used a previously validated categorisation as this was believed to best mirror the general use of the MDI, but it is possible that other cut-points would be preferable in this population. Smoothened linear plots of outcomes as a function of the MDI sum score gave no obvious reason to alter the categorisation. It was deemed necessary to combine mild to severe depression for the calculations of the predictive capacity of the MDI which may have lowered the accuracy. However, this did not affect the choice of questions to be used for a brief screening tool.

We presented results on combining items 1 and 3 with or (i.e. any of the two should be positive) which increased the accuracy of prediction activity limitation slightly. In order to increase the specificity of the screening questions it would appear logical to combine the two items by and (i.e. both should be positive), but with the applied cut-point nothing was achieved by that.

We chose to prioritise sensitivity when dichotomising the items since we consider an increased attention on potential psychological issues in patients for whom it is irrelevant less problematic than not being aware of depressive symptoms in those for whom it matters. The cut-point had no implications for the identification of the best screening items and could be altered if wanting a lower number of false positives.

In conclusion, signs of depression were infrequent in this LBP cohort seeking chiropractic care. Depression was associated with outcome but not a major explanation of poor prognosis. Single items of the MDI performed almost as good as prognostic factors as did the entire scale and items 1 or 3 can be used as alternatives to the total scale when a brief screening tool is needed. Head-to-head comparisons with other screening questions for depression are needed in order to recommend one rather than another. In short, depression is of importance in LBP when present but not prevalent in chiropractic practice. Therefore we recommend screening for depression with a very brief tool. This could be item 1 or item 3 from the MDI.