Setting and design
Saskatchewan is one of three Prairie Provinces in Western Canada. The majority of Saskatchewan residents receive provincial health care benefits. The rest of the population (< 1%) would include federally insured persons such as federal prison inmates, Royal Canadian Mounted Police (RCMP) and military personnel. These federally insured groups would have information captured in data collected around hospital use but not in other data sources employed in this study. Additionally, First Nations and Metis peoples whom have treaty relationships with the federal government, termed ‘Registered Indians’ (RI) and who comprise 15.6% (http://www12.statcan.gc.ca/nhs-enm/2011/as-sa/fogs-spg/Pages/FOG.cfm?lang=E&level=2&GeoCode=47) of the provincial population, receive some of their health benefits from the Federal Government. Relevant data from these populations with Federal treaty relationships was incorporated within the data sources employed in this study.
A Saskatchewan provincial population-based cohort study was undertaken to evaluate incidence and prevalence rates for RA between Fiscal Year 2001–02 (FY0102) and FY1415.
Subjects and data sources
This retrospective, population-based cohort study was achieved by employing the Saskatchewan Provincial Health Administrative Databases. All data available, spanning from April 1, 1996 to March 31, 2015, were used to identify the cohort. Provincial Health Administrative Databases utilized for this study included: the Discharge Abstract Database, the physician Medical Services Database, the Person Health Registration System, and the Vital Statistics Registry. These various data sources can be linked anonymously through unique personal health insurance numbers.
The Discharge Abstract Database incorporates detailed hospitalization related data. Until March 31, 2001, prior to the study time period, diagnoses were recorded in compliance with the International Classification of Diseases 9th revision (ICD-9). Subsequently, the International Classification of Diseases, 10th revision, Canadian Version (ICD-10-CA) was introduced. During the 12-month period from April 1, 2001 until March 31, 2002, approximately 90% of ICD codes recorded are ICD-10-CA with the remaining 10% being ICD-9. Subsequent to April 1, 2003 virtually all codes were recorded in the ICD-10-CA format. Between 3 and 16 diagnoses are captured in each record prior to the introduction of ICD-10-CA, and up to 25 diagnoses are captured subsequently. The database provides: detailed diagnostic information including the primary admission diagnosis, as well as co-morbidity diagnoses and diagnoses related to complications arising during the hospitalization.
The Medical Services Database provides data on physician services. Physicians who are paid on a fee-for-service basis submit billing claims to the provincial health ministry. A single diagnosis using three digit ICD-9 codes is recorded on each claim. Physicians who are salaried are generally required to submit billing claims for administrative purposes, a practice known as shadow billing.
The Person Health Registration System captures characteristics of each insured individual including their age, sex, location of residence, and dates of coverage within the provincial health insurance plan.
The Vital Statistics Registry holds information on all births and deaths within the province.
Cohort case definition
Rheumatoid arthritis
A previously validated algorithm for administrative data was employed in the identification of people with RA for this cohort [15]. Individuals were identified as having RA if they had three or more physician services claims for RA (ICD-9 code: 714), at least one of which was made by a rheumatologist, general internal medicine specialist or orthopedic surgeon, within a 2 year period, or if they had one or more hospitalizations with a diagnosis of RA (ICD-9 code: 714, ICD-10 codes: M05, M06) in any of the up to 25 diagnosis fields. When an individual met both the physician visit and hospitalization criteria, the earliest occurrence was taken as the index date of diagnosis. For inclusion in this cohort, individuals were required to be age 18 or older on the index date of their RA diagnosis and have uninterrupted health insurance coverage (i.e. a gap of no more than three consecutive days in coverage) from the date of their diagnosis until March 31, 2015 or their exit from the cohort.
To distinguish incident from prevalent RA, a lead-time of 2 years was used based on the clinical judgement that a person is unlikely to go more than 2 years without seeking medical attention for their newly developing RA. Individuals having 2 or more years of insurance coverage prior to their diagnosis of RA were identified as having incident RA. Anyone with less than 2 years of health insurance prior to their diagnosis with RA was included in the cohort to ensure prevalence estimates in subsequent years were accurate. They were included in incidence estimates, although they were flagged to determine the potential overestimation of the incidence of RA.
Death
The date of death was identified from the Vital Statistics Registry. An individual who died between April 1 and March 31 of the following year was described as having died in that fiscal year.
Co-variates
Descriptive variables were identified for each member of the cohort. All variables were determined on the day of RA diagnosis. These included: age (categories: 18- < 45, 45- < 55, 55- < 65, 65- < 75, 75 and more years), sex (male, female), insurance coverage (< 2 years, 2 and more years). Age, sex, and insurance coverage were obtained from the Person Health Registration System.
Statistical analysis
All analyses were performed at the Saskatchewan Health Quality Council using SAS© statistical software, version 9.3 (SAS Institute Inc. SAS. Cary, NC: SAS Institute Inc.; 2007). The FY0102 population data was used as the reference for directing standardization of subsequent years. Cell size count data of less than six was suppressed as per Saskatchewan Health Quality Council policy for protection of patient confidentiality.
The incidence rate of RA per 100,000 population at risk (PAR) was calculated for each fiscal year. As an example, in FY0102 the numerator is the number of people alive on April 1, 2001 who were diagnosed with RA between April 1, 2001 and March 31, 2002, inclusive. The denominator (i.e., the population at risk to develop RA) includes all individuals aged 18 years or older on April 1, 2001 with at least 1 day of health insurance coverage within the fiscal year after removing individuals with prevalent RA.
The prevalence rate of RA per 100,000 PAR was calculated for each fiscal year (FY). As an example, in FY0102, the numerator is the number of people alive on April 1, 2001 who were diagnosed with RA prior to April 1, 2001. The denominator includes all individuals aged 18 years or older on April 1, 2001 with at least 1 day of health insurance coverage within the fiscal year. Prevalent cases were carried forward each year unless they died or had a gap in their health coverage longer than 3 days.
Crude and adjusted annual incidence and prevalence rates and 95% confidence intervals (95% CI) were calculated for FY0102 to FY1415. Rates were adjusted to the age and sex distribution of the Saskatchewan population standard from fiscal year 2001/02. Annual rates were also stratified by sex and also by age category. Sex-stratified rates were standardized for age. Age category rates were standardized for sex.
This study was approved by the University of Saskatchewan Biomedical Research Ethics Board.
