© 2007 American Thoracic Society
Avoiding Mistakes in Calculating the Number Needed to Treat in Severe COPDFrom the Authors:We thank Dr. Aaron for the comments on our recent article (1), and for giving us the opportunity to clarify the issue of the number needed to treat (NNT) for exacerbations of chronic obstructive pulmonary disease (COPD). The NNT has been introduced for binary outcome variables as the reciprocal of the absolute risk reduction (2). However, our primary outcome (number of moderate and severe exacerbations) is not binary, since our patients with severe COPD had at least two (mean 2.87–2.91) exacerbations in the year preceding the inclusion into the study. From a clinically realistic point of view, we focused just on reducing the number of exacerbations (that is from three per year to two or one), even if they could not have been completely abolished. The NNT of 12.3 computed by Dr. Aaron says one has to treat on average 12.3 patients for 44 weeks to have 1 additional patient without any COPD exacerbation within 44 weeks. We, however, looked for the NNT to prevent one single exacerbation, since the NNT concept is not restricted to binary outcomes. There are extensions for other types of variables. A rather simple extension is possible for count variables—for example, number of COPD exacerbations within a given time period. Halpin defines "event-based NNTs," that is, the NNTs to avoid (or achieve) a particular outcome, as "the reciprocal of the difference between the treatment and the control groups in the rate of a particular outcome per patient within a given time frame" (3). For the event exacerbation one has to estimate the rate of exacerbations per patient and year in each treatment group and calculate the reciprocal of the difference. With the data from our study, this gives 1/(1.4 – 0.92) = 2.08. Avoiding one exacerbation, however, does not mean that the patient must have no exacerbation. This definition for an event-based NNT has been used by several authors (4–6).
Maingau Hospital, Frankfurt am Main, Germany
Frankfurt am Main, Germany
Hannover, Germany
Philipps University, Marburg, Germany FOOTNOTES
* M.W. and T.G. were employees of GlaxoSmithKline Germany at the time of the study. Conflict of Interest Statement: P.K. has been reimbursed by Altana, AstraZeneca (AZ), Cefalon, GlaxoSmithKline (GSK), Novartis, and Viatris for attending several conferences; he has participated as a speaker in scientific meetings and courses organized and financed by various pharmaceutical companies (Altana, AZ, Berlin-Chemic, Boehringer Ingelheim [BI], Cefalon, GSK, Novartis, and Pfizer) and served on advisory boards for the following companies: AZ, BI, GSK, Novartis, and Pfizer; he has received aggregated honoraria of $12,000 in 2005 from GSK and $11,000 from Novartis; and his group has received $14,000, $18,000, and $15,000 in 2003, 2004, and 2005, respectively, from Altana, AZ, GSK, and Novartis as research grants for participating in several multicenter clinical trials. M.W. was an employee of GlaxoSmithKline Germany at the time of the study. T.G. was an employee of GlaxoSmithKline Germany at the time of the study. C.V. has given presentations in the last three years at industry symposia sponsored by Altana, AstraZeneca (AZ), Bayer, Boehringer Ingelheim, GSK, Merck Darmstadt, Novartis, Pfizer, Sanofi Aventis, and Talecris; he also served on advisory boards for Altana, AZ, Bayer, Boehringer Ingelheim, GSK, Pfizer, and Sanofi Aventis; in addition, his institution took part in clinical studies sponsored by AZ, GSK, and Intermune; in 2005, aggregated honoraria and industry sponsored grants comprised $22,000 from Altana, $50,000 from AZ, and $30,000 from GSK. REFERENCES
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