Abstract: Four small studies (224 participants) were included in the review. Little information was provided about the process of allocating participants to groups. None of the studies used independent outcome assessors, and evidence suggested researcher allegiance towards the active treatments. The four studies examined a diversity of third wave CBT approaches (extended behavioural activation, acceptance and commitment therapy and competitive memory training) and control conditions. None of the studies conducted follow-up assessments. The results showed a significant difference in clinical response rates in favour of third wave CBT when compared with treatment as usual (TAU) conditions (three studies, 170 participants, risk ratio (RR) 0.51, 95% confidence interval (CI) 0.27 to 0.95; very low quality). No significant difference in treatment acceptability based on dropout rates was found between third wave CBT approaches and TAU (four studies, 224 participants, RR 1.01, 95% CI 0.08 to 12.30; very low quality). Both analyses showed substantial statistical heterogeneity. Very low quality evidence suggests that third wave CBT approaches appear to be more effective than treatment as usual in the treatment of acute depression. The very small number of available studies and the diverse types of interventions and control comparators, together with methodological limitations, limit the ability to draw any conclusions on their effect in the short term or over a longer term. The increasing popularity of third wave CBT approaches in clinical practice underscores the importance of completing further studies of third wave CBT approaches in the treatment of acute depression, on a short- and long-term basis, to provide evidence of their effectiveness to policy-makers, clinicians and users of services.

Plain-language summary: In this review, we focused on third wave CBT approaches, a group of psychological therapies that target the process of thoughts (rather than their content, as in CBT), helping people to become aware of their thoughts and to accept them in a non-judgemental way. The aim of the review was to find out whether third wave CBT was effective and acceptable to people in the acute phase of depression. The review included four studies, involving a total of 224 people. The studies examined three different forms of third wave CBT, consisting of extended behavioural activation (two studies), acceptance and commitment therapy (ACT) (one study) and another form of third wave CBT called competitive mind training (one study). Three of the studies compared third wave CBT approaches with treatment as usual control conditions. The fourth study compared ACT with a psychological placebo condition. The results suggested that third wave CBT approaches were effective on a short-term basis in treating depression. However, the quality of evidence was very low because of the small number of studies/participants included in the review, together with the diverse client groups, interventions and control conditions used and possible allegiance of researchers towards the active treatments, making it difficult to draw conclusions with any confidence. It is notable, too, that none of the studies looked at the long-term effect of third wave CBT approaches. Given the increasing popularity of third wave CBT approaches in clinical practice, further well-designed studies should be prioritised to establish whether third wave CBT approaches are helpful in treating people with acute depression.