First, the distinction between level 1 and level 2 depends in part on the level of evidence system you are using.

 

The CEBM criteria use level 1 for systematic reviews of randomized trials and also use level 1 for well-done randomized trials.  They further distinguish them as level 1a and level 1b.

 

The SORT criteria use level 1 for systematic reviews of randomized trials and also for well-done randomized trials.

 

Also keep in mind that a systematic review of lower-quality evidence should not imply that we have better evidence because a systematic search was done.

 

If the evidence we have is limited to lower-quality randomized trials (level 2), then a systematic review of those trials should also be considered level 2.  A high-quality systematic review can appropriately conclude level 2 evidence or insufficient evidence.

 

For the explicit criteria we use (http://www.ebscohost.com/dynamed/levels.php):

 

• DynaMed criteria for level 1 (likely reliable) evidence for a randomized trial
1. Full-text report available in English (or language well understood by participating editor)
2. Clinical outcome (also called patient-oriented outcomes)
3. Random allocation method (i.e. not assigned by date of birth, day of presentation, “every other”)
4. Allocation concealed
5. Blinding of all persons (patient, treating clinician, outcome assessor) if possible
6. Intention-to-treat analysis comparing groups according to randomization
7. Follow-up (endpoint assessment) of at least 80% of study entrants AND adequate such that losses to follow-up could not materially change results
8. Adequate statistical power
9. No other factors contributing substantial bias, such as
1. Differences in management between groups other than the intervention being studied
2. Differential loss to follow-up
3. Posthoc analysis
4. Subgroup analysis
5. Baseline differences between groups
6. Unclear how missing data is accounted for
• DynaMed criteria for level 1 (likely reliable) evidence for a systematic review
1. Full-text report available in English (or language well understood by participating editor)
2. Clinical outcome (also called patient-oriented outcomes)
3. Systematic search
4. Explicit inclusion criteria
5. Systematic selection of included studies
6. Evaluation of study quality
7. Additional criteria if meta-analysis
1. Studies are clinically appropriate for pooled analysis (reasonably similar populations, interventions, methodology and outcomes)
2. Meta-analysis not limited by statistically significant heterogeneity
8. Conclusion based on primary studies meeting Level 1 evidence criteria
9. Adequate statistical power
10. No other factors contributing substantial bias, such as
1. Inclusion criteria that appear to inappropriately exclude important evidence

2. Subgroup analysis
3. Indirect comparisons

 

 

What Liz says is sensible and I agree. How would classify a systematic review which incorporates a meta-anaylsis with cohort studies and case-control studies in the same stratum (it must have been very difficult to do, but the authors achieved it)? Or one which dodges the main question because the evidence to answer that question does not fit with the researchers' ideas or what the sponsor wants? Or a systematic review which ignores proven (published) evidence of suppressed safety data from trials in very small children?

They all exist in published format.

Tom.

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Dr Tom Jefferson
tel 0039 3292025051
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