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Pitfalls in nonrandomized outcomes studies: the case of incidental appendectomy with open cholecystectomy


Objectives — To assess the short-term outcomes of incidental appendectomy through analysis of hospital administrative data and determine the consistency and plausibility of the observed results.

Design — Population-based historical cohort study.

Setting — All general hospitals in Ontario between 1981 and 1990.

Patients — Patients undergoing open primary cholecystectomy with (7,846 exposed) and without (191,599 unexposed) incidental appendectomy.

Main Outcome Measures — In-hospital fatality rates, complication rates, and lengths of hospital stay.

Results — Crude comparisons showed a striking and paradoxical reduction in mortality after cholecystectomy when incidental appendectomy was performed (odds ratio [OR], 0.37; 95% confidence interval [CI], 0.23 to 0.57; P<.001); mean length of stay was also lower by -0.46 day (P<.001). After adjustment for confounding differences, such as comorbidity and nonelective surgery, mortality and lengths of stay were similar for exposed and unexposed patients; but exposed patients showed a significant increase in nonfatal complications (OR, 1.53; 95% CI, 1.39 to 1.68; P<.001). Adverse effects from incidental appendectomy emerged consistently for all three outcomes only after restricting the analysis to subgroups of patients at low surgical risk. The increased mortality for exposed patients was largest among low-risk groups; for example, among those younger than 70 years undergoing elective surgery, the OR was 2.65 (95% CI, 1.25 to 5.64; P<.001).

Conclusion — These findings suggest that incidental appendectomy is associated with a small but definite increase in adverse postoperative outcomes. However, plausible and consistent findings were only obtained after restricting the analysis to low-risk subgroups in which unmeasured differences in patients' baseline characteristics were less likely to confound adjusted outcome comparisons. This exercise highlights the potential pitfalls in nonrandomized outcomes comparisons using data sources with limited clinical detail, such as hospital discharge abstracts.



Wen SW, Hernandez R, Naylor CD. JAMA. 1995; 274(21):1687-91.

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