Original Research

Mismatch Between Process and Outcome Measures for Hospital-Acquired Venous Thromboembolism in a Surgical Cohort



From Tufts Medical Center, Boston, MA.


  • Objective: Audits at our academic medical center revealed near 100% compliance with protocols for perioperative venous thromboembolism (VTE) prophylaxis, but recent National Surgical Quality Improvement Program data demonstrated a higher than expected incidence of VTE (observed/expected = 1.32). The objective of this study was to identify potential causes of this discrepancy.
  • Design: Retrospective case-control study.
  • Setting: Urban academic medical center with high case-mix indices (Medicare approximately 2.4, non-Medicare approximately 2.0).
  • Participants: 102 surgical inpatients with VTE (September 2012 to October 2015) matched with controls for age, gender, and type of procedure.
  • Measurements: Prevalence of common VTE risk factors, length of stay, number of procedures, index operation times, and postoperative bed rest > 12 hours were assessed. Utilization of and compliance with our VTE risk assessment tool was also investigated.
  • Results: Cases underwent more procedures and had longer lengths of stay and index procedures than controls. In addition, cases were more likely to have had > 12 hours of postoperative bed rest and central venous access than controls. Cases had more infections and were more likely to have severe lung disease, thrombophilia, and a history of prior VTE than controls. No differences in body mass index, tobacco use, current or previous malignancy, or VTE risk assessment form use were observed. Overall, care complexity and risk factors were equally important in determining VTE incidence. Our analyses also revealed lack of strict adherence to our VTE risk stratification protocol and frequent use of suboptimal prophylactic regimens.
  • Conclusion: Well-accepted risk factors and overall care complexity determine VTE risk. Preventing VTE in high-risk patients requires assiduous attention to detail in VTE risk assessment and in delivery of optimal prophylaxis. Patients at especially high risk may require customized prophylactic regimens.

Keywords: hospital-acquired venous thromboembolic disease; VTE prophylaxis, surgical patients.

Deep vein thrombosis (DVT) and pulmonary embolism (PE) are well-recognized causes of morbidity and mortality in surgical patients. Between 350,000 and 600,000 cases of venous thromboembolism (VTE) occur each year in the United States, and it is responsible for approximately 10% of preventable in-hospital fatalities.1-3 Given VTE’s impact on patients and the healthcare system and the fact that it is preventable, intense effort has been focused on developing more effective prophylactic measures to decrease its incidence.2-4 In 2008, the surgeon general issued a “call to action” for increased efforts to prevent VTE.5

The American College of Chest Physicians (ACCP) guidelines subcategorize patients based on type of surgery. In addition, the ACCP guidelines support the use of a Caprini-based scoring system to aid in risk stratification and improve clinical decision-making (Table 1).4,6-9 In general, scores ≥ 5 qualify individuals as high risk. Based on their risk category, patients receive mechanical prophylaxis, chemical prophylaxis, or a combination of the 2. Lower-risk patients who are ambulatory typically receive only mechanical prophylaxis while in bed, whereas higher-risk patients receive a combination of mechanical prophylaxis and chemoprophylaxis measures.7 In general, low-molecular-weight heparin (40 mg daily) and low-dose unfractionated heparin (5000 units 3 times daily) have been the standard evidence-based options for chemoprophylaxis in surgical patients. Absolute contraindications for prophylaxis include active bleeding and known increased risk of bleeding based on patient- or procedure-specific factors.

Caprini Risk Assessment Model

Our hospital, a 350-bed academic medical center in downtown Boston, MA, serving a diverse population with a very high case-mix index (2.4 Medicare and 2.0 non-Medicare), has strict protocols for VTE prophylaxis consistent with the ACCP guidelines and based on the Surgical Care Improvement Project (SCIP) measures published in 2006.10 The SCIP mandates allow for considerable surgeon discretion in the use of chemoprophylaxis for neurosurgical cases and general and orthopedic surgery cases deemed to be at high risk for bleeding. In addition, SCIP requires only that prophylaxis be initiated within 24 hours of surgical end time. Although recent audits revealed nearly 100% compliance with SCIP-mandated protocols, National Surgical Quality Improvement Program (NSQIP) data showed that the incidence of VTE events at our institution was higher than expected (observed/expected [O/E] = 1.32).

In order to determine the reasons for this mismatch between process and outcome performance, we investigated whether there were characteristics of our patient population that contributed to the higher than expected rates of VTE, and we scrutinized our VTE prophylaxis protocol to determine if there were aspects of our process that were also contributory.


Study Sample

This is a retrospective case-control study of surgical inpatients at our hospital during the period September 2012 to October 2015. Cases were identified as patients diagnosed with a VTE (DVT or PE). Controls were identified from a pool of surgical patients whose courses were not complicated by VTE during the same time frame as the cases and who were matched as closely as possible by procedure code, age, and gender.


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