Volume 192 - Issue 12
Prevalence of venous thromboembolism in medical inpatients
Authors: J Alasdair Millar, Glenda E Lee and Rinaldo Ienco
Med J Aust 2010; 192 (12): 724-726. || doi: 10.5694/j.1326-5377.2010.tb03720.x
Published online: 21 June 2010
Published online: 21 June 2010
To the Editor: The clinical justification for a medical intervention depends on absolute prevalence (p) of a disease or condition in a population and the relative risk reduction (R) that would result from the intervention. These variables determine the “number needed to treat” (NNT) to prevent one occurrence of a disease, according to the formula NNT = [100 ÷ (P × R)], where P and R are expressed as percentages.
This principle applies to thromboprophylaxis in medical patients. However, the prevalence of venous thromboembolism (VTE) in hospitalised patients is uncertain. The main justification for medical thromboprophylaxis given on the National Institute of Clinical Studies (NICS) website1 is an unpublished report prepared by the University of Western Australia (UWA) on behalf of the NICS.2 The report noted that 40.8% of all hospital cases of VTE were “medical” rather than “surgical” or “idiopathic” (primary) cases, but the prevalence of VTE (overall or in each subgroup) was not stated.
We obtained coded separation data for all multiday admissions to Royal Perth Hospital (RPH) for the most recent 2-year period with complete data (2005–2007). We searched for VTE events with a principal or secondary coding, and classified them as medical or surgical VTE cases according to the definitions used in the UWA report (for medical VTE, “admissions in which a diagnosis of VTE was recorded as a complication or in a diagnostic field other than the principal diagnosis OR admissions with VTE as the principal diagnosis within 3 months of a non-surgical [medical] admission”).2 We acknowledge that the use of prophylaxis during the index admission or any previous admission within 3 months (which we did not measure) may have meant that we underestimated the prevalence of VTE. Prevalence was calculated by dividing the event number by the total number of admissions or by the number of medical or surgical admissions, as required (expressed as a percentage).
At RPH over the 2-year period, 805 VTE events (574 in medical and 231 in surgical patients) were observed in 72 991 medical and 29 177 surgical admissions (total, 102 168 admissions). These included 357 pulmonary emboli (44.3%), 207 of which were primary events. Of the 805 VTE events, 312 (38.8%) were medical, 209 (26.0%) surgical and 284 (35.3%) idiopathic (the corresponding proportions in the UWA report were 40.8%, 37.7% and 21.5%, respectively). The overall VTE rate in the medical patient population was 0.79%, but for “medical VTE” as defined in the UWA report,2 the rate was 0.43%. In surgical patients, the rate of VTE was 0.79% in our study, or 0.72% based on the UWA definition. Hence, according to the definitions used by the UWA report, “surgical VTE” is actually more frequent than “medical VTE”.
Our study confirms that about 40% of VTE is in medical patients, but that the absolute prevalence is low (0.43%). This is similar to the rate of 0.4% reported in the PREVENT (Prevention of Recurrent Venous Thromboembolism) study.3 The 40% figure is not relevant for consideration of prophylaxis, as it depends on the number of non-medical events. The low prevalence is directly relevant, and weakens the case, as previously argued, for routine thromboprophylaxis.4
In summary, the NICS support for medical thromboprophylaxis may be biased by its reliance on the UWA report. The risk is of overuse of drugs that cause bleeding, and hence of doing more harm than good.5
References
- National Institute for Clinical Studies. Venous thromboembolism prevention program — the evidence base. http://www.nhmrc.gov.au/nics/programs/vtp/venous.htm (accessed Dec 2009).
- National Institute of Clinical Studies. The incidence and risk factors for venous thromboembolism in hospitals in Western Australia 1999–2001. Report commissioned from the School of Population Health, University of Western Australia. Canberra: NICS, 2005. http://www.nhmrc.gov.au/nics/material_resources/resources/incident_risk.htm (accessed Dec 2009).
- Leizorovicz A, Cohen AT, Turpie AG, et al; PREVENT Medical Thromboprophylaxis Study Group. Randomized, placebo-controlled trial of dalteparin for the prevention of venous thromboembolism in acutely ill medical patients. Circulation 2004; 110: 874-879. 0_CACEBIHD
- Millar JA. Rationale for thromboprophylaxis in medical inpatients; not quite there yet. Med J Aust 2008; 189: 504-506. 0_CACDCIHJ
- Millar JA. Selection of medical patients for prophylaxis of venous thromboembolism based on analysis of the benefit-hazard ratio. Intern Med J 2009; 39: 606-612. 0_i1091877