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CONTINUING MEDICAL EDUCATION FORMATION MÉDICALE CONTINUE USERS’ GUIDE TO THE SURGICAL LITERATURE How to work with a subgroup analysis Bernadette Dijkman, BSc* urgical practice should principally be based on evidence originating Bauke Kooistra, BSc* Mohit Bhandari, MD, MSc*† for the Evidence-Based Surgery S from high-quality data such as randomized controlled trials (RCTs). Whereas these studies mostly investigate general and representative patient populations, clinical decisions most often depend on individual patient characteristics. To concede to the need of individually based guidelines, many Working Group‡ RCTs report analyses on specific subgroups of patients.1,2 The main aim of a subgroup analysis is to identify either consistency of or large differences in the From the ‡Surgical Outcomes Research Centre, Department of Clinical Epidemi- magnitude of treatment effect among different categories of patients. Deter- ology, McMaster University, and the mining whether the observed overall treatment effect is different across cer- *Division of Orthopaedic Surgery, tain subgroups may justly provide some patients with its benefits and protect McMaster University, Hamilton, Ont. others from its harm. Irrespective of its practical potentials, subgroup analysis must be conscien- ‡The Evidence-Based Surgery Working Group comprises Drs. S. Archibald, tious in design, reporting and interpretation. Many stringent methodological F. Baillie, M. Bhandari, M. Cadeddu, criteria apply but are far from always fulfilled.2 Consequently, inferences C. Cinà, F. Farrokhyar, C.H. Goldsmith, drawn may wrongfully direct management of certain patient groups. In fact, T. Haines, R. Hansebout, R. Jaeschke, the definition of a subgroup analysis is equivocal in that authors use the term C. Levis, M. Simunovic, V. Tandan, and A. Thoma and Ms. S. Cornacchi and to indicate tests that estimate differences in treatment effect within subgroups Ms. A. Garnett. (a subgroup effect) and between subgroups (an interaction; Table 1). The purpose of this article is to consider criteria for sound subgroup analy- Accepted for publication ses in RCTs, assuming good underlying methodological quality of the main Feb. 10, 2009 trial (i.e., randomization, assessor blinding, etc.).3 A clinical scenario, based on a recent RCT in orthopedic surgery, will practically support the theoretical Correspondence to: Dr. M. Bhandari statements throughout the text. Division of Orthopaedic Surgery McMaster University CLINICAL SCENARIO 293 Wellington St. N, Ste. 110 Hamilton ON L8L 2X2 fax 905 523-8781 A 25-year-old woman keeps returning to your practice with recurrent anter- firstname.lastname@example.org ior dislocations of her shoulder. Since her initial dislocation more than 3 years ago, she has had 3 recurrent dislocations and several subluxations of her shoulder. On the second dislocation, you tried a different method of reduction and immobilized her shoulder for a longer time, but unfortunately this did not prevent another redislocation. You noticed that some patients in your practice with dislocated shoulders did not have any recurrences, despite receiving exactly the same treatment. So you asked yourself, “What makes this patient different from the others?”, and you searched the literature to find an answer. Age of the patient4–6 and duration of immobilization7,8 might explain the difference in recurrence, but the data remain largely inconclusive. You recall a colleague discussing immobilization of the shoulder in external rotation (ER) rather than the usual internal rotation (IR) as a great method to reduce recurrences. You decide to expand your search to identify the best available evidence on shoulder position. LITERATURE SEARCH To find out if internal rotation immobilization has ever been compared with another immobilization method, you search the available literature. You perform © 2009 Canadian Medical Association Can J Surg, Vol. 52, No. 6, December 2009 515 FORMATION MÉDICALE CONTINUE a comprehensive search9 using the following search terms: (p = 0.033)
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