Su Golder and colleagues carry out an overview of meta-analyses to assess whether
estimates of the risk of harm outcomes differ between randomized trials and
observational studies. They find that, on average, there is no difference in the
estimates of risk between overviews of observational studies and overviews of
randomized trials.
Background
There is considerable debate as to the relative merits of using randomised
controlled trial (RCT) data as opposed to observational data in systematic
reviews of adverse effects. This meta-analysis of meta-analyses aimed to
assess the level of agreement or disagreement in the estimates of harm
derived from meta-analysis of RCTs as compared to meta-analysis of
observational studies.
Methods and Findings
Searches were carried out in ten databases in addition to reference checking,
contacting experts, citation searches, and hand-searching key journals,
conference proceedings, and Web sites. Studies were included where a pooled
relative measure of an adverse effect (odds ratio or risk ratio) from RCTs
could be directly compared, using the ratio of odds ratios, with the pooled
estimate for the same adverse effect arising from observational studies.
Nineteen studies, yielding 58 meta-analyses, were identified for inclusion.
The pooled ratio of odds ratios of RCTs compared to observational studies
was estimated to be 1.03 (95% confidence interval 0.93–1.15).
There was less discrepancy with larger studies. The symmetric funnel plot
suggests that there is no consistent difference between risk estimates from
meta-analysis of RCT data and those from meta-analysis of observational
studies. In almost all instances, the estimates of harm from meta-analyses
of the different study designs had 95% confidence intervals that
overlapped (54/58, 93%). In terms of statistical significance, in
nearly two-thirds (37/58, 64%), the results agreed (both studies
showing a significant increase or significant decrease or both showing no
significant difference). In only one meta-analysis about one adverse effect
was there opposing statistical significance.
Conclusions
Empirical evidence from this overview indicates that there is no difference
on average in the risk estimate of adverse effects of an intervention
derived from meta-analyses of RCTs and meta-analyses of observational
studies. This suggests that systematic reviews of adverse effects should not
be restricted to specific study types.
Please see later in the article for the Editors' Summary
Editors' Summary
Background
Whenever patients consult a doctor, they expect the treatments they receive
to be effective and to have minimal adverse effects (side effects). To
ensure that this is the case, all treatments now undergo exhaustive clinical
research—carefully designed investigations that test new treatments
and therapies in people. Clinical investigations fall into two main
groups—randomized controlled trials (RCTs) and observational, or
non-randomized, studies. In RCTs, groups of patients with a specific disease
or condition are randomly assigned to receive the new treatment or a control
treatment, and the outcomes (for example, improvements in health and the
occurrence of specific adverse effects) of the two groups of patients are
compared. Because the patients are randomly chosen, differences in outcomes
between the two groups are likely to be treatment-related. In observational
studies, patients who are receiving a specific treatment are enrolled and
outcomes in this group are compared to those in a similar group of untreated
patients. Because the patient groups are not randomly chosen, differences in
outcomes between cases and controls may be the result of a hidden shared
characteristic among the cases rather than treatment-related (so-called
confounding variables).
Why Was This Study Done?
Although data from individual trials and studies are valuable, much more
information about a potential new treatment can be obtained by
systematically reviewing all the evidence and then doing a meta-analysis
(so-called evidence-based medicine). A systematic review uses predefined
criteria to identify all the research on a treatment; meta-analysis is a
statistical method for combining the results of several studies to yield
“pooled estimates” of the treatment effect (the efficacy of a
treatment) and the risk of harm. Treatment effect estimates can differ
between RCTs and observational studies, but what about adverse effect
estimates? Can different study designs provide a consistent picture of the
risk of harm, or are the results from different study designs so disparate
that it would be meaningless to combine them in a single review? In this
methodological overview, which comprises a systematic review and
meta-analyses, the researchers assess the level of agreement in the
estimates of harm derived from meta-analysis of RCTs with estimates derived
from meta-analysis of observational studies.
What Did the Researchers Do and Find?
The researchers searched literature databases and reference lists, consulted
experts, and hand-searched various other sources for studies in which the
pooled estimate of an adverse effect from RCTs could be directly compared to
the pooled estimate for the same adverse effect from observational studies.
They identified 19 studies that together covered 58 separate adverse
effects. In almost all instances, the estimates of harm obtained from
meta-analyses of RCTs and observational studies had overlapping 95%
confidence intervals. That is, in statistical terms, the estimates of harm
were similar. Moreover, in nearly two-thirds of cases, there was agreement
between RCTs and observational studies about whether a treatment caused a
significant increase in adverse effects, a significant decrease, or no
significant change (a significant change is one unlikely to have occurred by
chance). Finally, the researchers used meta-analysis to calculate that the
pooled ratio of the odds ratios (a statistical measurement of risk) of RCTs
compared to observational studies was 1.03. This figure suggests that there
was no consistent difference between risk estimates obtained from
meta-analysis of RCT data and those obtained from meta-analysis of
observational study data.
What Do These Findings Mean?
The findings of this methodological overview suggest that there is no
difference on average in the risk estimate of an intervention's adverse
effects obtained from meta-analyses of RCTs and from meta-analyses of
observational studies. Although limited by some aspects of its design, this
overview has several important implications for the conduct of systematic
reviews of adverse effects. In particular, it suggests that, rather than
limiting systematic reviews to certain study designs, it might be better to
evaluate a broad range of studies. In this way, it might be possible to
build a more complete, more generalizable picture of potential harms
associated with an intervention, without any loss of validity, than by
evaluating a single type of study. Such a picture, in combination with
estimates of treatment effects also obtained from systematic reviews and
meta-analyses, would help clinicians decide the best treatment for their
patients.
Additional Information
Please access these Web sites via the online version of this summary at
http://dx.doi.org/10.1371/journal.pmed.1001026.
The US National Institutes of Health provide information on clinical
research; the UK National Health Service Choices Web
site also has a page on clinical trials and medical research
The Cochrane
Collaboration produces and disseminates systematic
reviews of health-care interventions
Medline Plus provides links to further information about clinical trials (in English and Spanish)