Multiple sources of data are needed for accurate reporting of blood clots following surgery

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The journal Thrombosis Research has published a new study that used multiple sources of data to look at rates of blood clots or venous thromboembolism (VTE) occurring after surgery among veterans who were hospitalized for surgeries during the period 2005 to 2010. VTE is an important and preventable health problem that is more common after surgery. Better quality data on how often VTE occurs is needed to determine the success of efforts to prevent VTE in veterans and other patients. You can read the article’s abstract here.

While several studies have used hospital discharge data to estimate numbers of VTEs occurring in surgical patients prior to hospital discharge, in many cases VTE occurs after a patient leaves the hospital. Administrative healthcare data (billing data) cover large patient populations, but it is unclear how many patients for whom VTE is reported were actual cases. It can be difficult from billing data to tell the difference between someone who just developed a clot for the first time from someone who may be having symptoms of a previous unresolved clot or someone who is being treated to prevent a clot from developing. Researchers from the University of Utah, the Veterans Affairs Salt Lake City Health Care System and CDC combined multiple types of information to more accurately estimate rates of VTE in patients occurring within 30 days and 90 days of having a hospital-based surgery. The study used both administrative data and a method used to access and scan medical care notes from patient electronic health records (EHR) to identify cases of VTE occurring after hospital discharge.


What is Venous Thromboembolism (VTE)?
Venous thromboembolism includes both deep vein thrombosis (DVT) and pulmonary embolism (PE), and is an important and growing public health issue.

What is Deep Vein Thrombosis (DVT)?
Deep Vein Thrombosis is a condition in which blood clots form in the deep veins of the body, typically in the legs, although sometimes in the arms.

What is Pulmonary Embolism (PE)?
Pulmonary Embolism occurs when a clot in a deep vein breaks free and travels through the heart to block arteries of the lungs.

Main Findings from this Study

  • Among almost 500,000 surgeries performed in Department of Veterans Affairs (VA) hospitals from 2005 to 2010, roughly 1 in 100 patients developed a VTE within 90 days. Approximately 6 in 10 VTEs following surgery occurred up to 90 days after the patient left the hospital. Using hospital discharge data alone to track how often VTE occurs after surgery leads to an understatement of the true size and scope of the problem.
  • The study identified presumed outpatient diagnoses of VTE that were based on billing codes and filled prescriptions for blood thinners (medicines to treat VTE), and then used a computerized analysis of written physician notes contained in EHRs to confirm the outpatient diagnoses of VTE. Among patients who within 90 days of discharge had an outpatient VTE diagnosis according to administrative records (billing codes) and filled a prescription for a blood thinner, just 1 of 4 had a new VTE diagnosis confirmed by physician notes. These results suggest that the frequency of new VTE diagnoses after patients are discharged from the hospital is actually not as high as it would appear based only on administrative records of billing codes and filled prescriptions for blood thinners.
  • Among VTEs that were recorded within 90 days of surgery,
    • About 4 in 10 occurred in patients prior to hospital discharge, and
    • About 6 in 10 were diagnosed after discharge. Among these patients,
      • Most (80%) were readmitted to a VA hospital, while
      • Only 20% were not readmitted to a VA hospital.
  • The ability of the VA patient care database to track patients over time across multiple healthcare settings is crucial to assessing the frequency of these blood clotting events.
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CDC study shows that about 1 in 100 patients developed a life-threatening blood clot after surgery. Learn more.

About this Study

Study researchers analyzed data from a national patient care database covering more than 3 million hospitalizations at 111 VA facilities from 2005 through 2010. VTE events were identified through different processes. First, hospital records were searched for the presence of 1 or more of selected International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) codes indicating a diagnosis of DVT or PE or both together with a hospital pharmacy record for blood thinners. ICD-9-CM is the official system of assigning codes to diagnoses and procedures associated with hospital care in the United States. Second, outpatient records (after hospital discharge) were searched for a relevant ICD-9-CM code together with a filled prescription for a blood thinner. For outpatient records that met both of these criteria, the diagnosis of a new VTE was called “confirmed” if a written physician note documented the diagnosis in the EHR.

More Information

Paper Reference

Nelson RE, Grosse SD, Waitzman NJ, Lin J, DuVall SL, Patterson O, Tsai J, Reyes N. Using multiple sources of data for surveillance of postoperative venous thromboembolism among surgical patients treated in Department of Veterans Affairs hospitals. Thrombosis Research. 2015 Apr;135(4):636-42