Data Sources and Data Linkage
Administrative datasets housed at the ICES were used to obtain the data
for this study. ICES is an independent, non-profit research organization
funded by the Ontario Ministry of Health and Long Term Care whose data
holdings include all health care related events for the population of
Ontario. The Registered Persons Database (RPDB) is a repository of
demographic information for all residents of who are eligible for the
Ontario Health Insurance Plan (OHIP). RPDB is maintained by the
Ministry, and contains information on health card number, date of birth,
sex, address, and deceased date (where applicable). Personal identifying
information in RPDB is removed at ICES, and each unique health number is
converted into an anonymous unique identifier, the ICES Key Number
(IKN). The IKN is a common identifier that is used to link data sources
within ICES. The OHIP database contains data on the diagnosis and
fee-for-service claims submitted by physicians and paid by the universal
health care system. The Canadian Institute for Health Information
Discharge Abstract Database and Same Day Surgery database contain
information on all inpatient and day surgery visits, respectively,
abstracted from hospital records for patient diagnoses and procedures.
The datasets were created at ICES Queen’s based on patients of all ages,
using physician OHIP billing for an outpatient anesthesia consult (OHIP
fee code: A015, A016) (Table 1) from April 1, 2002 to June 30, 2018, and
hospital discharge records from April 1, 2002 to September 30, 2018
(study period). Patients were excluded if they had missing age/sex data,
or were a resident of another province at the time of consultation or
surgery. These datasets were then linked to demographic data, and to
hospital inpatient and outpatient discharge data for surgeries.
The study cohort included all patients who underwent an outpatient
preoperative anesthesia consultation (A015, A016) during the study
period. The index date was the first use of these OHIP billing claims.
This cohort permitted reporting of counts and percentages of surgery,
and for those patients who underwent surgery, the ASA classification
(American Society of Anesthesiologists physical status classification
system) of the patients by the anesthesiologist at the time of surgery.
The ASA classification is used ubiquitously to stratify the general
health of patients whereby an ASA I patient is healthy with no medical
diagnoses, all the way up to an ASA V patient who is so medically
complex that he or she is not expected to survive for 24 hours with or
without surgical intervention (Table 2)11. For ASA
III, IV, and V patients, there are respective anesthesia modifier
billing codes through OHIP. This is due to the increased complexity of
providing perioperative anesthesia care for patients with increased
comorbidities. There are no modifier ASA billing codes for patients who
are deemed ASA I or ASA II (healthier) at the time of surgery by the
attending anesthesiologist. As such, if a modifier for ASA III, IV, or V
was not used, the patient was presumed to be ASA I or II.