Table 1 Systematic reviews included: main characteristics and findings

Edwards SJ

1998

14

Various studies suggest that giving people more information and time to reflect tends to be associated with a lower consent rate. There seems to be an optimal level of information about side-effects such that patients are not overburdened by detail. More information in general is associated with greater awareness of the research nature of the trial, voluntariness of participation, right to withdraw and alternative treatments. This result does not extend to explanations of randomization on which the literature is contradictory. High levels of knowledge are significantly associated with less anxiety, irrespective of consent method. The more patients know before they are invited to participate in a trial, the better equipped they are to cope with the informed consent procedure

Flory J

2004

30 on 42 trials

Multimedia interventions: of 12 trials; only 1 published and 2 unpublished documented an improvement in understanding. Of 15 trials of enhanced consent forms, 6 showed significant improvement in understanding but 5 of these 6 were of limited quality

Of 5 trials of extended discussion, 3 showed significant improvement in understanding and 2 showed trends toward improvement. Of 5 trials of test/feedback, all showed significant improvement in understanding but were flawed (may mistook rote memorization for improvement in understanding). Another 5 trials were put into a miscellaneous category and had varying impact on understanding. Some demographic factors, particularly lower education, were associated with less understanding. Satisfaction and willingness to enrol were never significantly diminished by an intervention

Falagas ME

2009

30 related to clinical trials

A high level of understanding of the aim of the clinical trial was reported by 83–100% of the participants in 14 studies. In 4 out of 8 studies reporting data on understanding randomization, 91–100% of participants had a high level of understanding the meaning of randomization. The concept of voluntarism was highly understood by 81% to 100% of the participats in 7 of 15 studies, and so was the concept of withdrawal (more than 81% of the participants in 7 of 16 trials). The potential complications and risks during participation in clinical trials were highly understood by 90% to 100% of the participants in 8 of 16 studies. In 1 of 15 studies, 85% of participants in the clinical trials seemed to expect they would be successfully treated

Mandava A

2012

47

No substantial difference between participants in developing and developed countries in their understanding of the trial purpose. Reported understanding of side effects varied widely, depending on how the questions were framed; understanding randomization and placebo-controlled designs was generally lower than understanding of other aspects; a clear difference between participants in developing and developed countries was related to knowledge of the right to refuse to participate or withdraw (less in developing countries)

Palmer BW

2012

20

10 studies reported that multimedia form consent was associated with significantly better understanding of information; 6 reported partial benefits; 4 reported no significant difference between multimedia and standard consent process

Nilsen ES

2013

6

There is low-quality evidence that an informed consent document developed with consumer input may have little if any impact on understanding compared to a consent document developed by trial investigators only. There is low-quality evidence that consumer consultation in the development of consent documents may have little if any impact on: participant’s self-reported understanding of the trial described in the consent document; satisfaction with study participation; adherence to the protocol; refusal to participate

Nishimura A

2013

39 on 54 interventions

Meta-analysis of multimedia approaches was associated with a non-significant increase in understanding (SMD 0.30, 95% CI − 0.23 to 0.84); enhanced consent form, with significant increase (SMD 1.73, 95% CI 0.99 to 2.47); and extended discussion, with significant increase (SMD 0.53, 95% CI 0.21 to 0.84). By review, 31% of multimedia interventions showed significant improvement in understanding; 41% for an enhanced consent form; 50% for extended discussion; 33% for test/feedback; and 29% for miscellaneous. Multiple sources of variation: control processes, the presence of a human proctor, real vs. simulated protocol, and assessment formats

Tamariz L

2013

6

Included 1,620 research participants. The specific intervention differed in each study. Two included the teach-back method or teach-to-goal method and achieved the highest level of comprehension. Two studies changed the readability level of the IC and resulted in the lowest comprehension among study subjects. Interventions where a study team member spent more time talking one-on-one to study participants were the most effective at improving their understanding

Montalvo w

2014

27

Participants' understanding was limited. Most studies (78%) used investigator-developed tools to assess participant understanding, did not assess participants’ health literacy (74%), or did not assess the readability of the consent form (89%). Participants lacked basic understanding of research elements: randomization, placebo, risks, and therapeutic misconception

Synnot A

2014

16

The value of audio-visual interventions remains largely unclear, although trends are emerging with regard to improvements in knowledge and satisfaction with the information. There is not enough evidence to draw conclusions about anxiety arising from audio-visual informed consent. There is conflicting, very low-quality evidence about whether audio-visual interventions take more or less time to administer, and no study measured researcher satisfaction with the informed consent process, or ease of use

Gillies K

2015

1 on 2 trials

290 randomised participants. Effects on knowledge and decision conflict are uncertain. Decision regret: small effect (low quality) in favor of the decision aids (MD -5.53, 95% CI − 10.29 to − 0.76). There was uncertainty about any effect on enrolment and attrition (no. of participants who dropped out of the parent RCT who were enrolled in the D Aid trial). Anxiety: uncertain effect

Grootens-Wiegers P

2015

20

Multimedia improved understanding in 3 studies, particularly when information was customised, and seemed ineffective in one. Another study investigated the effect of different presentations of probability information and found that children understood pie charts and percentages best; in one study, children preferred a story format to a standard or question-and-answer format; in another, children preferred an enhanced form with bullet points, bold type, larger font size and illustrations, over a standard form. There is a gap between the required reading level and the average reading level of children

Tam NT

2015

103

135 cohorts of participants. The pooled proportion of participants who understood components of informed consent was 75.8% for freedom to withdraw at any time, 74.7% for the nature of study, 74.7% for the voluntary nature of participation, 74.0% for potential benefits, 69.6% for the study’s purpose, 67.0% for potential risks and side-effects, 66.2% for confidentiality, 64.1% for the availability of alternative treatment if withdrawn, 62.9% for knowing that treatments were being compared, 53.3% for placebo and 52.1% for randomization, 62.4% had no therapeutic misconceptions and 54.9% could name at least one risk. Covariates such as age, educational level, critical illness, study phase and location significantly affected understanding and indicated that the proportion of participants who understood informed consent had not increased in 30 years

Kao CY

2017

9

All studies with methodological limitations, reported mixed effects. Inconsistent effects of interventions on participants' knowledge and understanding of clinical trials. However, satisfaction with the interventions, or with decision-making regarding whether to participate in a trial was high overall