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Editor's Correspondence
July 26, 1999

Evidence-Based Evaluation of Preoperative vs Postoperative Use of Low-Molecular-Weight Heparin in Preventing Deep Vein Thrombosis in Elective Hip Surgery

Arch Intern Med. 1999;159(14):1622-1623. doi:

The recent report by Hull et al1 describing the preoperative vs postoperative initiation of low-molecular-weight heparin (LMWH) prophylaxis in patients undergoing elective hip replacement provides an excellent example of the clinical usefulness of a meta-analysis. The number needed to treat (NNT) is a term that is gaining importance in assessing therapy. It is defined as the number of patients needed to treat to achieve 1 favorable outcome.2 The number needed to treat is calculated as l divided by the absolute risk reduction and is a better measure of clinical significance than relative risk reduction and statistical significance.3 One other strength of NNT is that it can also be defined to describe adverse effects of treatment (ie, number needed to harm [NNH]).