This study was conducted at Burnaby Hospital, a 267-bed urban hospital in southwestern British Columbia. The 2008 ACCP guidelines were used in formulating the data collection form, which stratified patients according to the type of surgery they had undergone and their individual risk factors. The patients were stratified into 4 groups (designated 0, 1, 2, and 3) according to their additional risk factors for venous throm- boembolism (see Appendix 1). These categories were used to determine if patients at higher risk for venous thromboem- bolism received an extended duration of thromboprophylaxis (> 10 days). An observational analysis was conducted to determine the duration of therapy (up to 10 days, 10—27 days, or more than 27 days) for patients undergoing each type of surgery (total hip or knee replacement or hip fracture surgery). The rationale for this analysis stemmed from the ACCP guide- lines, which recommended that all patients receive thrombo- prophylaxis for at least 10 days, along with a strong suggestion that thromboprophylaxis be continued for up to 35 days. Clinical trials have studied extended-duration prophylaxis ranging from 28 to 35 days. For the current study, the intention was to determine if the duration of thromboprophy- laxis was variable and if any such variation could be related to the type of surgery.

The project received approval from the Research Ethics Board of the Fraser Health Authority before initiation of data collection.
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To be included in the study, patients had to be older than 18 years of age and had to have undergone total hip or knee replacement or hip fracture surgery at the Burnaby Hospital during the defined study period of March 30, 2007, to April 1, 2008. The Decisions Support Department provided a list of all patients meeting these inclusion criteria. Based on published adherence rates of 50%, a sample size of 200 patients was necessary to have 95% confidence in choosing a sample of patient charts that would adequately represent all patients who underwent total hip or knee replacement or hip fracture surgery during the study period.

Patients were excluded if they had been taking anticoagu­lants before admission or if they had a bleeding disorder that would contraindicate the use of anticoagulants (hemorrhagic diathesis, severe thrombocytopenia, suspected intracranial hemorrhage, active ulcerative lesions, or hemophilia).

The sample cohort of patients was selected using a stratified systematic sampling method. A total of 114 total hip replace­ments, 258 total knee replacements, and 138 hip fracture surgeries had been performed at the hospital during the study period. To ensure a cohort of patients that would be represen­tative of the entire sample, it was determined that the sample should consist of about 67 patients who had undergone total hip replacement, 73 patients who had undergone total knee replacement, and 70 patients who had undergone hip fracture surgery, assuming at least 50% adherence to the guidelines. Subsequent to the stratified systematic sampling, the final cohort consisted of 194 patients. To select patients who had undergone total hip replacement or hip fracture surgery, every second patient was selected from the respective lists of patients who had undergone these procedures during the study period. To select patients who had undergone total knee replacement, every fourth patient was selected from the list of patients had undergone total knee replacement during the study period. The principal investigator (M.A.M.) reviewed the charts of the selected patients to determine the duration of thromboprophy- laxis after surgery but before discharge from the hospital. Information about the agent used in the hospital was collected, and risk factors were assessed (see Appendix 1).
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Information about thromboprophylaxis after discharge from the hospital was obtained from PharmaNet, a province- wide network that holds a central database of all prescription medications dispensed in British Columbia. These data reflected the duration of use of thomboprophylactic medication dispensed from community pharmacies after hospital discharge. Following the chart review, a cohort file including specified patient information (personal health number, number of days of thromboprophylaxis in the hospital, index date [date of discharge], code for surgery type, code for risk level, and thromboprophylactic agent used in the hospital) was sent to PharmaNet. PharmaNet staff then calculated each study patient’s total duration of thromboprophylaxis, by summing the days of therapy in the hospital and the days of therapy in the community (i.e., days’ supply of anticoagulants represented by prescriptions filled after discharge). To preserve patients’ confidentiality, the cohort file was then stripped of the patients’ personal health number and the duration of thromboprophy- laxis received in the hospital, and the calculated data were returned to the investigators. Information received back from PharmaNet consisted of a study identification number (to replace each patient’s personal health number), total duration of thromboprophylaxis therapy (days), surgery code, risk code, index date (date of discharge), anticoagulant used, quantity of medication, and days supply. With the information provided by PharmaNet, it was possible to determine the rate of adherence to the 2008 ACCP guidelines for duration of thromboprophylaxis. It was also possible to determine the trends in prescribing for patients with various levels of risk associated with these procedures. Descriptive statistics were used to summarize the results.