Contact: Patrick Phillips, Patrick.phillips@sheffield.ac.uk School Of And Health Related Research Volume-outcome relationships in the treatment of AAA in Europe: a systematic review. Phillips, P; Poku, E; Essat, M; Woods, HB; Goka, EA; Kaltenthaler, EC; Shackley, P; Michaels, JA. School of Health and Related Research (ScHARR), University of Sheffield, S1 4DA, UK Background: Searches for evidence of a relationship between the volume of patients treated for an abdominal aortic aneurysm (AAA) and outcomes identified nine relevant systematic reviews which were generally supportive of an inverse relationship for mortality. However the evidence base was low quality, dated and more relevant to the context of the USA than Europe. Subsequent scoping searches identified a number of relevant primary studies from Europe which were not included in the identified reviews, thus establishing a need for a new systematic review in this area. The objective of this review was to evaluate the relationship between the volume of AAA surgery undertaken by individual clinicians or hospitals and outcomes (primary outcome mortality). A publicly available and pre-registered protocol is available at: http://www.crd.york.ac.uk/PROSPERO/display_record.asp?ID=CRD42014014850 Literature Searching: Comprehensive electronic searches were conducted in multiple databases, including MEDLINE and EMBASE to March 2015, using MESH and free text terms, these searches were supplemented by reference list and citation searches. Inclusion criteria: studies, published in the last ten years, of European populations of adults undergoing elective or emergency abdominal aortic aneurysm treatment where the effect of hospital or operator volume on outcomes is reported. Data Extraction was conducted independently by two authors, quality assessment was conducted using a modified version of ACROBAT-NRSI. Synthesis by meta-analysis was planned in the event that there was sufficient clinical, methodological and statistical homogeneity in included studies; alternatively a narrative synthesis was planned. 14,486 individual references identified following de-duplication 14,038 records excluded after title and abstract screening 417 papers excluded following full text scrutiny 31 potentially eligible full text papers 17 excluded as carotid or lower limb vascular populations 14 studies included in the AAA review Results: fourteen eligible studies (n =200,506 participants) were included; 10 from the UK, 3 from Germany and 1 from Norway. All the included studies were observational using a combination of prospective, retrospective, administrative and clinical data. There was a high degree of heterogeneity in terms of clinical and procedural groups investigated which is compounded by; variety in the methods used to define volume groups, in the analysis of outcomes and by different studies using overlapping data sources making meta-analyses inappropriate. Overall, included studies were of low methodological quality, particularly in relation to the risk of selection bias. Preliminary synthesis of adjusted and unadjusted data suggests a relationship between high volumes of procedures conducted in hospitals and reduced mortality. The relationship between surgeon volume and mortality and other relevant outcomes could not be established due to insufficient evidence. Conclusion: Current evidence suggests the existence of an inverse relationship between the volume of AAA procedures conducted in hospital and mortality, though this conclusion is tentative based on the relatively low quality of the evidence. There is insufficient evidence to reach conclusions regarding other outcomes and relationships. Funding and disclaimer: This poster presents independent research funded by the National Institute for Health Research (NIHR) under the Programme Grants for Applied Research programme (RP-PG-1210-12009). The views expressed are those of the authors and not necessarily those of the NHS, the NIHR or the Department of Health Contact: Patrick Phillips, Patrick.phillips@sheffield.ac.uk School of Health and Related Research (ScHARR), University of Sheffield, S1 4DA, UK www.facebook.com/scharrsheffield ruth.wong@sheffield.ac.uk