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CMAJ • September 30, 2003; 169 (7)
© 2003 Canadian Medical Association or its licensors

Letters
Correspondance

No absolutes

Daniel Hackam

Division of Clinical Pharmacology, Department of Medicine, University of Toronto, Toronto, Ont.

Anthony Kerrigan states that relative risk reduction does not take into account primary and secondary end points (which are expressed by absolute risk reduction) and that presenting absolute risk reductions would give practitioners more realistic estimates of the benefits of specific interventions. Although this viewpoint has been frequently expressed in letters to the editor and editorials, its premise is flawed.

Patients enrolled in clinical trials are frequently at lower risk of important adverse outcomes than the patients commonly encountered in actual practice, many of whom have risk-increasing comorbidities that tend to exclude them from such studies. Therefore, the absolute risk reduction reported in a pertinent clinical trial cannot readily be applied to such patients. The obvious solution is to calculate a new absolute risk reduction (and a new number needed to treat) based on the relative risk reduction reported in the clinical trial, as applied to the patient's estimated baseline, pretreatment risk.1 Fortunately, the relative risk reductions derived from cardiovascular trials tend to be relatively impervious to the baseline risk of the patient. Therefore, as implied by my summary2 of the Anglo-Scandinavian Cardiac Outcomes Trial (ASCOT),3 it is entirely appropriate for practitioners to apply the relative, not the absolute, risk reduction from such clinical trials to the patients they see.

Daniel Hackam Division of Clinical Pharmacology Department of Medicine University of Toronto Toronto, Ont.

References

  1. Furukawa TA, Guyatt GH, Griffith LE. Can we individualize the number needed to treat (NNT)? An empirical study of summary effect measures in meta-analyses. Int J Epidemiol 2002; 31: 72-6.[Abstract/Free Full Text]
  2. Hackman DG. Do hypertensive patients with average cholesterol levels benefit from atorvastatin therapy? CMAJ 2003;168(13):1689.[Free Full Text]
  3. ASCOT investigators. Prevention of coronary and stroke events with atorvastatin in hypertensive patients who have average or lower-than- average cholesterol concentrations, in the Anglo-Scandinavian Cardiac Outcomes Trial — Lipid Lowering Arm (ASCOT-LLA): a multicentre randomised controlled trial. Lancet 2003;361: 1149-58.[Medline]




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