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Centre for Evidence-
Based Medicine

Harm: Clinical Scenario

You are a nurse practitioner working in a family practice setting. One of your patients is a 28-year old woman who has been taking oral contraceptives (OCs) for the past 6 years. She telephones you to say that she is growing increasingly anxious about taking OCs because recently she has read in magazines and heard from friends that they may increase the risk of cardiovascular disease. She has never smoked, is in good health, and has no history of cardiovascular disease in her family. She asks to see you to discuss whether she should discontinue taking the OCs and begin using a different birth control method.

You admit to yourself that it has been some time since you reviewed the literature on this topic and at that time, most of the studies were case controls. You recall from an editorial in Evidence-Based Nursing (1999;2:4-6) that case control studies are weaker than randomized controlled trials or cohort studies because they begin with the outcome of interest (e.g., cardiovascular disease) and look back for exposure to the causative agent (e.g., OCs). The limitations of this retrospective design are the difficulties in: establishing that the exposure actually occurred before the outcome (temporality); obtaining accurate information about exposure to a causative agent that occurred in the past (relies on accuracy of people's memory or on completeness and accuracy of medical records); and identifying a control group that is similar in all other factors that may have influenced the outcome.

In preparation for your appointment with your patient, you decide to formulate a question and search for prospective studies. You formulate the question, "In women taking oral contraceptives, is there an association between their use and cardiovascular disease?"

Searching terms and evidence source:
You begin an advanced search on PubMed using the search terms "oral contracept*" AND "cardiovascular disease" AND "risk factor*". Combining these 3 terms yields a total of 832 citations. To further narrow your search, you enter the terms "cohort stud*" AND "prospective" (because you have learned that for studies of harm, prospective cohort studies have fewer methodological problems than case-control or cross-sectional studies) - 10 citations are identified. The first citation you look at seems promising -- Mant J, Painter R, Vessey M. Risk of myocardial infarction, angina and stroke in users of oral contraceptives: an updated analysis of a cohort study. Br J Obstet Gynaecol 1998;105:890-6.

Read the article and decide:

  1. Is the evidence from this randomised trial valid?
  2. If valid, is this evidence important?
  3. If valid and important, can you apply this evidence in caring for your patient?

Continue to completed worksheet