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Incidence of venous thromboembolism in users of combined oral contraceptives

BMJ 2000; 320 doi: (Published 01 January 2000) Cite this as: BMJ 2000;320:57

Methods for identifying cases and estimating person time at risk must be detailed

  1. Hershel Jick, associate professor of medicine
  1. Boston Collaborative Drug Surveillance Program, Boston University School of Medicine, 11 Muzzey Street, Lexington, MA 02421, USA
  2. Leiden University Medical Centre, PO Box 9600, 2300 RC Leiden, Netherlands
  3. European Institute of Health and Medical Sciences, University of Surrey, Guildford GU2 5RF

    EDITOR—Lawrenson et al reported a crude incidence of venous thromboembolism of 38 per 100 000 women who used combined oral contraceptives, based on information derived from the General Practice Research Database, without describing how they derived their estimate.1 This estimate is closely similar to the incidence reported in a prior publication of theirs, which was based on a different automated medical database.2 In their letter they provide no details on the validity and specificity of the diagnosis of venous thromboembolism or the presence of medical risk factors and no information on the person time at risk for current users of oral contraceptives.

    An incidence must be based on person time at risk. Despite apparent deficiencies, the authors provide an incidence that is roughly twice as high as the incidence that colleagues and I reported in a paper derived from the General Practice Research Database; we included only idiopathic cases of venous thromboembolism (those possibly related directly to use of oral contraceptives).3 Lawrenson et al conclude that our reported estimate was substantially lower than the true incidence.

    On the basis of their previous publication,2 Lawrenson et al seem to have included both outpatients and inpatients with any one of five computer recorded diagnoses as patients with venous thromboembolism, without documentation from clinical records. The five diagnoses include a computer recorded diagnosis of thrombophlebitis. Colleagues and I found that, on the General Practice Research Database, over 90% of subjects with this computer recorded …

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