The incidence of venous thromboembolism: a prospective, community-based study in Perth, Western Australia

The study population comprised all residents of an area of about 94 km2 in north-eastern metropolitan Perth (defined by 12 postcodes: 6000, 6003, 6004, 6006, 6050–6054, 6059, 6060 and 6062). This represented an expansion of the area covered by the PCSS, which included eight complete postcode districts and part of a ninth.6 Royal Perth Hospital, a public tertiary referral centre, is located toward the southern apex of the study area, and another public tertiary centre, Sir Charles Gairdner Hospital, is situated just outside of the area to the west.

Data on resident population by sex and age were obtained from the Australian Bureau of Statistics 2001 national census. We defined residents as those who had lived, or intended to live, in the study area for ≥ 6 months. Residents who experienced VTE while temporarily away from the area were included in the study, but visitors to Perth were excluded.

To ensure case ascertainment was as complete as possible, we searched for incident cases prospectively (“hot pursuit”) during the study period and retrospectively (“cold pursuit”) during and after the study period.

Prospectively, we identified cases of VTE in hospital patients from computerised inpatient and emergency department attendance registers, lists of patients undergoing radiological tests, and lists of patients attending the Royal Perth Hospital Thrombosis Clinic. Patients with VTE in the community were prospectively ascertained by inviting general practitioners and privately operated radiological services to refer patients to the study, and by reviewing lists of patients in domiciliary nursing programs. We also placed advertisements in community newspapers and the newsletter of a home nursing organisation so that potentially suitable patients could refer themselves to the study.

Retrospectively, hospital patients were identified by searching the hospital morbidity and mortality databases of the Western Australian Department of Health. All public and private hospitals in the Perth area are required to submit hospital discharge data to the Department of Health, where they are regularly collated. We retrieved data on VTE hospitalisations using ICD-10 (International classification of diseases, 10th revision) codes (Box 1).7

Sudden deaths in the community were ascertained through the Coroner’s Court of Western Australia and its forensic pathology department. We also reviewed autopsy lists at Royal Perth Hospital and Sir Charles Gairdner Hospital, as they performed all postmortem examinations for inhospital deaths not subject to a coronial inquiry on behalf of community general hospitals in the study area.

We reviewed all patients’ medical records and, wherever possible, interviewed the patients to ensure that only symptomatic, objectively verified index events were included and that patients were residents of the study area.

To be counted as a case of VTE, symptomatic patients required objective confirmation of VTE by current standard diagnostic imaging or pathological confirmation of a clot removed during surgery or autopsy that was judged to have caused or contributed to the patient’s symptoms (or death, in the case of PE). We accepted the diagnosis of VTE reported by radiologists or nuclear physicians; reports were not reviewed or adjudicated. We included all cases of thrombosis of the deep veins of the limbs (proximal and distal) and abdominal viscera, but excluded cases of thrombophlebitis of the superficial veins of the limbs without thrombus extension into the deep veins. Where objectively confirmed symptomatic embolism to the lung(s) coexisted with DVT, we considered the event to be PE.

VTE events were classified as “first ever” or recurrent, based primarily on results of previous radiological investigations. We considered DVT or PE in a patient with neither DVT nor PE previously confirmed as first ever, but coded first-ever DVT in a patient with previous objectively verified PE (and vice versa) as recurrent.

When previous medical records and radiology reports were unavailable, and there was no satisfactory clinical or diagnostic tool to confirm a previous diagnosis of VTE, we applied two sets of questions validated for use in epidemiological studies8 to ascertain: (1) if patients thought they had ever had a VTE, if they had ever been hospitalised for a PE, or if a physician had ever diagnosed them with a VTE; and (2) if they had ever received anticoagulation therapy. We considered patients answering in the affirmative to both sets of questions to have had previous VTE, and the index event was then counted as a recurrence. This approach has a sensitivity of 37.1%, specificity of 99.4%, and positive and negative predictive values of 48.9% and 98.5%, respectively.8

The VTE event rate was the total number of symptomatic, objectively verified VTE events that occurred per 1000 residents in the study area per year. Individual patients could have multiple separate events during the study period (eg, recurrences).

Crude annual incidence of VTE was calculated as the number of patients with symptomatic, objectively verified VTE per 1000 residents in the study area per year.

Age-adjusted annual incidence of VTE was the number of patients with symptomatic, objectively verified VTE per 1000 residents in the study area per year adjusted (according to age) to the World Health Organization World Standard Population,9 calculated by direct standardisation.10

We calculated 95% confidence intervals around estimates using exact limits.11

We obtained ethics approval for the study from all five tertiary referral hospitals in metropolitan Perth, and community and private hospitals serving the north-eastern metropolitan area (see Acknowledgements).

During the study period, there were 140 VTE events among 137 patients (Box 2). Sixty-six patients with VTE (48.2%) were identified during hospital presentation or at the Royal Perth Hospital Thrombosis Clinic, and 22 (16.1%) were identified through review of radiology lists. Thirty-nine patients (28.5%) were ascertained from the hospital morbidity and mortality databases; four (2.9%) from domiciliary nursing programs; two (1.5%) by referral to the study by other nurses; two (1.5%) by referral from a private radiological service; one (0.7%) from the Coroner’s Court; and one patient (0.7%) identified himself to the study after seeing a local advertisement.

The 140 VTE events comprised 87 DVT events and 53 PE events. The annual VTE event rate was 0.85 (95% CI, 0.71–0.99) per 1000 residents.

The crude annual incidence per 1000 residents was 0.83 (95% CI, 0.69–0.97) for VTE (Box 2), 0.52 (95% CI, 0.41–0.63) for DVT, and 0.31 (95% CI, 0.22–0.40) for PE. The annual incidences per 1000 residents adjusted to the WHO World Standard Population were 0.57 (95% CI, 0.47–0.67), 0.35 (95% CI, 0.26–0.44) and 0.21 (95% CI, 0.14–0.28) for VTE, DVT and PE, respectively.

Of the 137 patients, 102 (74.5%) had their first-ever thrombosis event. The incidences of first-ever VTE, DVT and PE were 0.62 (95% CI, 0.50–0.74), 0.38 (95% CI, 0.29–0.47) and 0.24 (95% CI, 0.17–0.31) per 1000 residents per year, respectively.

There was only one case of VTE among patients aged < 20 years; in this age group, the annual incidence was 0.03 (95% CI, 0–0.09) per 1000 residents. Among patients aged ≥ 80 years, the annual incidence was 5.36 (95% CI, 3.59–7.13) per 1000 residents (Box 3).

The mean age of patients with VTE was 64.6 years (range, 19–90 years). Among patients with DVT, the mean age was 64.4 years, and for those with PE, 64.9 years. The mean age of patients with first-ever VTE was similar to those whose thrombosis was recurrent (64.7 and 64.4 years, respectively).

There was a male preponderance of VTE (1.14 : 1) due to a higher incidence of DVT in men; PE incidence was equal between the sexes. In patients with DVT, there were three cases of thrombosis of the mesenteric veins, one of the portal vein, four in the upper limbs, and the remaining 78 in the lower limbs.