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Frequency of adverse events in patients with poor anticoagulation: a meta-analysis

From the Department of Medicine, University of Ottawa (all authors); the Clinical Epidemiology Program (Oake, Fergusson, Forster), Ottawa Health Research Institute, Ottawa; and the Institute for Clinical Evaluative Science (van Walraven), Toronto, Ont.

Correspondence to: Natalie Oake, Clinical Epidemiology Program, Ottawa Health Research Institute, Room C414, Ottawa Hospital, Civic Campus, 1053 Carling Avenue, Ottawa ON K1Y 4E9; fax 613 761-5492; noake{at}ohri.ca

Background: Patients taking anticoagulants orally over the long term have international normalized ratios (INRs) outside the individual therapeutic range more than one-third of the time. Improved anticoagulation control will reduce hemorrhagic and thromboembolic event rates. To gauge the potential effect of improved anticoagulation control, we undertook to determine the proportion of anticoagulant-associated events that occur when INRs are outside the therapeutic range.

Methods: We conducted a meta-analysis of all studies that assigned hemorrhagic and thromboembolic events in patients taking anticoagulants to discrete INR ranges. We identified studies using the MEDLINE (1966–2006) and EMBASE (1980–2006) databases. We included studies reported in English if the majority of patients taking oral anticoagulants had an INR range with a lower limit between 1.8 and 2 and an upper limit between 3 and 3.5, and their INR at the time of the hemorrhagic or thromboembolic event was recorded.

Results: The final analysis included results from 45 studies (23 that reported both hemorrhages and thromboemboli; 14 that reported hemorrhages only; and 8, thromboemboli only) involving a median of 208 patients (limits of interquartile range [25th–75th percentile] 131–523 subjects; total n = 71 065). Of these studies, 64% were conducted at community practices; the remainder, at anticoagulation clinics. About 69% of the studies were classed as having moderate or high quality. Overall, 44% (95% confidence interval [CI] 39%–49%) of hemorrhages occurred when INRs were above the therapeutic range, and 48% (95% CI 41%–55%) of thromboemboli took place when below it. The mean proportion of events that occurred while the patient's INR was outside the therapeutic range was greater for studies with a short mean follow-up (< 1 yr). Between-study heterogeneity was significant (p < 0.001).

Interpretation: Improved anticoagulation control could decrease the likelihood of almost half of all anticoagulant-associated adverse events.

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