Can AMSTAR also be applied to systematic reviews of non-randomized studies?

BACKGROUND: There is a lack of an instrument to evaluate systematic reviews of non-randomized studies in epidemiological research. The Assessment of Multiple Systematic Reviews (AMSTAR) is widely used to evaluate the scientific quality of systematic reviews, but it has not been validated for SRs of non-randomized studies. The objective of this paper is to report our experience in applying AMSTAR to systematic reviews of non-randomized studies in terms of applicability, reliability and feasibility. Thus, we applied AMSTAR to a recently published review of 32 systematic reviews of non-randomized studies investigating the hospital volume-outcome relationship in surgery. RESULTS: The inter-rater reliability was high (0.76), albeit items 8 (scientific quality used in formulating conclusions), 9 (appropriate method to combine studies), and 11 (conflicts of interest) scored moderate (≤0.58). However, there was a high heterogeneity between the two pairs of reviewers. In terms of feasibility, AMSTAR proved easy to apply to systematic reviews of non-randomized studies, each review taking 5-10 minutes to complete. We faced problems in applying three items, mainly related to scientific quality of the included studies. CONCLUSIONS: AMSTAR showed good psychometric properties, comparable to prior findings in systematic reviews of randomized controlled trials. AMSTAR can be applied to systematic reviews of non-randomized studies, although there are some item specific issues users should be aware of. Revisions and extensions of AMSTAR might be helpful.