Figure 1 shows the process of study selection for our review. We excluded six potentially eligible studies because numbers of participants were below 50, five because they were published before 2000, and four on both criteria. We identified 29 comparisons as being potentially eligible. There was moderate evidence of asymmetry in the funnel plot for these studies (Egger’s regression test intercept −2.4 (SE 0.8), P=0.007, fig 2). We gained access to data from 16 (55%) of these comparisons, with data unavailable either because of no response from authors (n=8), clashes with their own planned analyses (n=4), or ethical issues with sharing data (n=1). A small number of individual cases were dropped because of missing baseline age or depression scores, leaving 2470 unique cases, with 77% reporting data at first follow-up. Group allocation had the strongest association with missing follow-up data, with patients in the usual care group less likely to have missing outcome data. Such patterns of missing data might be expected to result in an inflation of the overall effect (if missing data was associated with poor outcomes), but the effect on the interaction is difficult to predict.
Fig 1Inclusion of studies in the review
Fig 2Funnel plot of studies included in analysis with pseudo 95% confidence intervals. Egger’s regression intercept −2.39 (SE 0.8), P=0.007
Available and unavailable data
Data on study characteristics and design are detailed in the “Additional resources” file on bmj.com. We compared available and unavailable studies on population, intervention, quality, and outcome data (see table). Studies were similar in recruitment procedures, although available studies were less likely to involve patients with a diagnosis of depression or health technologies delivered via information technology, but were more likely to involve support from a health professional. Available studies met more quality criteria, had a slightly larger sample size, and reported lower estimates of effect.
Comparison of available and unavailable studies. Values are numbers (percentages) unless stated otherwise
Baseline characteristics of patients included in the review
As noted earlier, patients participating in low intensity trials are selected to be appropriate for these interventions, so we assessed the severity of depression of patients included in these trials. Six studies (38%) had a maximum ceiling for inclusion. Assessment of mean depression scores at baseline showed that many patients had appreciable symptoms (see fig 3). For the BDI score (range 0–63), a score of 10–16 indicates mild depression, 17–29 indicates moderate depression, and ≥30 indicates severe depression: the studies’ mean scores were 19–21,44
For the CES-D score (range 0–60), a score of ≥16 indicates a probable depressive illness, and the studies’ mean scores ranged from 13 in a trial focussed on subthreshold symptoms53
Fig 3Baseline severity data of studies included in the review. Box and whisker plots show median, interquartile range, minimum and maximum scores, and outliers
In terms of other characteristics of the patients, comparisons are limited by the data presented and reflect study inclusion criteria, but generally two thirds to three quarters of patients were women, with mean ages 35–45 years, and with rates of university education ranging from 20% to 65%. In terms of treatment history, rates of current antidepressant use (where reported) ranged from 19% to 69%, and between 38% and 67% reported a previous treatment for depression.
Is the effect of low intensity interventions on depression moderated by baseline depression severity?
The overall standardised estimate of the main effect of low intensity interventions was −0.42 (95% confidence interval −0.55 to −0.29, I2=2.9% (0.5% to 15%)). This would be equivalent to an additional drop of around four or five points on both BDI and CES-D scores, over and above the change in the controls. There was no evidence that this main effect varied by age, sex, intervention type, or study quality. When a term was added to assess the interaction, we found a significant negative interaction between baseline severity and treatment effect (interaction coefficient −0.1 (−0.19 to −0.002)). This suggests that patients who are more severely depressed at baseline demonstrate larger treatment effects than those who are less severely depressed. However, the magnitude of the interaction is small. As scores had been standardised, the effect represented an additional standardised benefit of 0.1 for an increase in initial severity of one standard deviation, which would be equivalent to an additional drop of around one point on both BDI and CES-D for a one standard deviation increase in initial severity, an effect which may not be clinically significant. The interpretation of the main result is outlined in clinical terms in box 2.
Figure 4 shows the estimates of the interactions at the level of the individual studies. The estimate was similar when conducted on available data without imputation (−0.12 (95% confidence interval −0.22 to −0.02)) and was not sensitive to variation in the meta-analytic model specified or the different measures included in the trials (BDI or CES-D score).
Fig 4Forest plot of interactions between baseline severity of depression and effect of low intensity interventions
Box 2: Clinical scenario
Patients attending primary care and considered eligible for psychological therapy for depression may present with a Beck Depression Inventory (BDI) score of around 25 (out of a maximum of 63), indicating moderate severity of depression. After three to six months in usual primary care, without any intervention, such patients might be expected to reduce their score on average by four points to around 21, still indicative of moderate depression.
If such patients were referred to a low intensity intervention, they might be expected to display an additional reduction of four points on average, over and above this natural change over time, to a score of around 17, indicative of milder depression.
The evidence presented in this paper would suggest that patients who present with more severe problems (such as a presenting score of 35) would show an additional drop of around one point (a total reduction of around five points) compared with those with an initial score of 25.
The results are displayed visually below. The horizontal axis shows initial severity of depression, and the vertical axis shows severity at follow-up. As can be seen from fig 5, patients in the low intensity intervention group consistently demonstrate lower severity of depression at follow-up than usual care patients. These lower scores are evident across the entire range of initial depression severity (that is, the lines never cross). The additional benefit shown by patients treated with low intensity interventions increases as initial severity increases (that is, the vertical distance between the lines increases as initial depression severity increases). However, the magnitude of this divergence is relatively small and is unlikely to be clinically significant.
Fig 5How baseline severity of depression moderates the effect of low intensity interventions on depression
The data illustrate that:
- (a) low intensity interventions provide clinically significant benefits over usual care
- (b) patients with more severe problems show greater levels of benefit from low intensity interventions, although the magnitude of that additional benefit is modest and may not be clinically significant.
Although patients with more severe depression show greater benefits over usual care, their initial high scores mean that they are more likely to continue to show clinically important levels of distress after low intensity interventions and may require additional care.
Is there a moderating effect of baseline depression severity at higher levels of depression?
The main analysis reported in the previous section showed a small but significant increase in effect of low intensity interventions in patients with more severe depression at presentation. When data were analysed in terms of five severity subgroups, we observed a stepwise increase in the effect of low intensity interventions, from least to most severely ill patients, but there was no statistically significant difference in the effect across the groups. Thus there was no indication that patients at the highest levels of severity showed different effects to the overall trend.
Are the results sensitive to allocation concealment?
The moderating effect of initial depression was larger in patients in studies with adequate concealment of allocation, but the difference was not statistically significant (interaction coefficient −0.07 (95% confidence interval −0.34 to 0.21)).
Are the results sensitive to types of low intensity interventions?
The moderating effect of initial depression was larger in patients in the studies that used internet based low intensity interventions, compared with the studies that used written interventions, but the difference was not statistically significant (interaction coefficient −0.09 (−0.31 to 0.12)). The moderating effect of initial depression was also greater in patients who used unguided low intensity interventions, compared with those who used guided interventions, but again the difference was not significant (interaction coefficient −0.07 (−0.30 to 0.15)).