Understanding administrative abdominal aortic aneurysm mortality data

Administrative data in the form of Hospital Episode Statistics (HES) and the Scottish Morbidity Record (SMR) have been used to describe surgical activity. These data have also been used to compare outcomes from different hospitals and regions, and to corroborate data submitted to national audits and registries. The aim of this observational study was to examine the completeness and accuracy of administrative data relating to abdominal aortic aneurysm (AAA) repair.

Administrative data (SMR-01 returns) from a single health board relating to AAA repair were requested (September 2007 to August 2012). A complete list of validated procedures; termed the reference data set was compiled from all available sources (clinical and administrative). For each patient episode electronic health records were scrutinised to confirm urgency of admission, diagnosis, and operative repair. The 30-day mortality was recorded. The reference data set was used to systematically validate the SMR-01 returns.

The reference data set contained 608 verified procedures. SMR-01 returns identified 2433 episodes of care (1724 patients) in which a discharge diagnosis included AAA. This included 574 operative repairs. There were 34 missing cases (5.6%) from SMR-01 returns; nine of these patients died within 30 days of the index procedure. Omission of these cases made a statistically significant improvement to perceived 30-day mortality (p < .05, chi-square test). If inconsistent SMR-01 data (in terms of ICD-10 and OPCS-4 codes) were excluded only 81.9% of operative repairs were correctly identified and only 30.9% of deaths were captured.

The SMR-01 returns contain multiple errors. There also appears to be a systematic bias that reduces apparent 30-day mortality. Using these data alone to describe or compare activity or outcomes must be done with caution.