Using days alive and out of hospital to measure surgical outcomes in New Zealand: a cross-sectional study

OBJECTIVES: To measure differences at various deciles in days alive and out of hospital to 90 days (DAOH(90)) and explore its utility for identifying outliers of performance among district health boards (DHBs). METHODS: Days in hospital and mortality within 90 days of surgery were extracted by linking data from the New Zealand National Minimum Data Set and the births and deaths registry between 1 January 2011 and 31 December 2021 for all adults in New Zealand undergoing acute laparotomy (AL—a relatively high-risk group), elective total hip replacement (THR—a medium risk group) or lower segment caesarean section (LSCS—a low-risk group). DAOH(90) was calculated without censoring to zero in cases of mortality. For each DHB, direct risk standardisation was used to adjust for potential confounders and presented in deciles according to baseline patient risk. The Mann-Whitney U test assessed overall DAOH(90) differences between DHBs, and comparisons are presented between selected deciles of DAOH(90) for each operation. RESULTS: We obtained national data for 35 175, 52 032 and 117 695 patients undergoing AL, THR and LSCS procedures, respectively. We have demonstrated that calculating DAOH without censoring zero allows for differences between procedures and DHBs to be identified. Risk-adjusted national mean DAOH(90) Scores were 64.0 days, 79.0 days and 82.0 days at the 0.1 decile and 75.0 days, 82.0 days and 84.0 days at the 0.2 decile for AL, THR and LSCS, respectively, matching to their expected risk profiles. Differences between procedures and DHBs were most marked at lower deciles of the DAOH(90) distribution, and outlier DHBs were detectable. Corresponding 90-day mortality rates were 5.45%, 0.78% and 0.01%. CONCLUSION: In New Zealand after direct risk adjustment, differences in DAOH(90) between three types of surgical procedure reflected their respective risk levels and associated mortality rates. Outlier DHBs were identified for each procedure. Thus, our approach to analysing DAOH(90) appears to have considerable face validity and potential utility for contributing to the measurement of perioperative outcomes in an audit or quality improvement setting.