Andrew Liu, BSc Hon, BScPhm, RPh, Clinical Pharmacist, Thrombosis Service, Toronto East General Hospital, Toronto, ON.
Carmine Stumpo, BScPhm, PharmD, RPh, Director, Pharmacy and Emergency Services, Toronto East General Hospital, Toronto, ON.

Warfarin-drug interactions are often encountered in the care of older adults. Interactions may be classified as pharmacokinetic, resulting in changes in serum warfarin concentrations, or pharmacodynamic, resulting in changes in hemostasis or platelet function. Knowledge of these mechanisms of warfarin-drug interactions may help identify warfarin interactions, facilitate prescribing decisions, and assist with appropriate monitoring.
Key words: warfarin, drug interactions, anticoagulants, cytochrome P-450 enzyme system, older adults.

Jeff Silverman, MD, FRCPC, Fellow in Adult Hematology, University of Toronto, Toronto, ON.
William Geerts, MD, FRCPC, Consultant in Clinical Thromboembolism, Sunnybrook and Women's College Health Sciences Centre, University of Toronto, Toronto, ON.

Introduction
Thrombocytopenia, defined as a platelet count of less than 150 x 10⁹/L, is an important clinical problem most commonly encountered in hospitalized patients. Although the differential diagnosis is extensive (Table 1), it is essential to always consider heparin-induced thrombocytopenia (HIT) in patients with thrombocytopenia who are hospitalized or who have recently been in a hospital.^1,2

HIT is an adverse drug reaction induced by exposure to heparin that is followed by thrombocytopenia, platelet activation and a dramatic increase in thrombosis risk. Although it is one of the most common and serious drug reactions in hospitalized patients, HIT is frequently not recognized until a major thromboembolic complication has resulted. However, if diagnosed and treated promptly, the outcome is generally favourable. With the widespread use of heparin in the elderly, geriatric patients constitute the largest population at risk of developing HIT. Therefore, clinicians providing care for the elderly must be able to recognize and manage HIT effectively and efficiently.

D'Arcy L. Little, MD, CCFP
York Community Services, Toronto, ON

Introduction
Every year there are approximately 50,000 strokes in Canada. Currently, close to 300,000 Canadians are stroke survivors. As stroke is an age-related condition, the number of strokes is predicted to increase as the Canadian population ages. The resultant national cost, which is estimated at 2.7 billion annually, will also increase unless improvements are made to prevention and treatment.¹ Approximately 1 in 6 survivors of a first stroke experiences a recurrent stroke over the next 5 years, of which 25% are fatal within 28 days.² The above statistics suggest that attention to secondary stroke prevention would be important in reducing the morbidity, mortality and cost to society of stroke. The purpose of this article is to review the role of anti-platelet and anticoagulant agents in the secondary prevention of stroke.

Goals of Therapy
Therapeutic measures in secondary stroke prevention aim to prevent recurrent stroke or transient ischemic attacks, with the aim of preventing morbidity and mortality from incremental neurological deficits, as well as preventing associated cardiac ischemic events.