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Can J Anaesth. 2013 Jul;60(7):675-83. doi: 10.1007/s12630-013-9935-5. Epub 2013 Apr 19.

Predictors of unanticipated admission following ambulatory surgery: a retrospective case-control study.

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Department of Anesthesia, McMaster University, Hamilton, ON, Canada.



The primary objectives of this historical case-control study were to evaluate the incidence of and reasons and risk factors for adult unanticipated admissions in three tertiary care Canadian hospitals following ambulatory surgery.


A random sample of 200 patients requiring admission (cases) and 200 patients not requiring admission (controls) was taken from 20,657 ambulatory procedures was identified and compared. The following variables were included: demographics, reason for admission, type of anesthesia, surgical procedure, length of procedure, American Society of Anesthesiologists' (ASA) classification, surgical completion time, pre-anesthesia clinic, medical history, medications (classes), and perioperative complications. Multiple logistic regression analysis was used to assess factors associated with unanticipated admissions.


The incidence of unanticipated admission following ambulatory surgery was 2.67%. The most common reasons for admission were surgical (40%), anesthetic (20%), and medical (19%). The following factors were found to be associated with an increased risk of unanticipated admission: length of surgery of one to three hours (odds ratio [OR] 16.70; 95% confidence interval [CI] 4.10 to 67.99) and length of surgery more than three hours (OR 4.26; 95% CI 2.40 to 7.55); ASA class III (OR 4.60; 95% CI 1.81 to 11.68); ASA class IV (OR 6.51; 95% CI 1.66 to 25.59); advanced age (> 80 yr) (OR 5.41; 95% CI 1.54 to 19.01); and body mass index (BMI) of 30-35 (OR 2.81; 95% CI 1.31 to 6.04). Current smoking status was found to be associated with a decreased likelihood of unanticipated admission (OR 0.44; 95% CI 0.23 to 0.83), as was monitored anesthesia care when compared with general anesthesia (OR 0.17; 95% CI 0.04 to 0.68) and plastic (OR 0.18; 95% CI 0.07 to 0.50), orthopedic (OR 0.16; 95% CI 0.08 to 0.33), and dental/ear-nose-throat surgery (OR 0.32; 95% CI 0.13 to 0.83) when compared with general surgery. Other comorbid conditions did not impact unanticipated admission.


Unanticipated admission after ambulatory surgery occurs mainly due to surgical, anesthetic, and medical complications. Length of surgery more than one hour, high ASA class, advanced age, and increased BMI were all predictors. No specific comorbid illness was associated with an increased likelihood of unanticipated admission. These findings support continued use of the ASA classification as a marker of patient perioperative risk rather than attributing risk to a specific disease process.

[Indexed for MEDLINE]

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