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BMJ. Mar 19, 2005; 330(7492): 672.
PMCID: PMC554954

Managing osteoarthritis of the knee

Conclusions about use of NSAIDs are misleading
Florence Tubach, assistant professor of epidemiology
INSERM U738; Groupe Hospitalier Bichat-Claude Bernard (Assistance Publique—Hôpitaux de Paris); Faculté Xavier Bichat (Université Paris 7), 46 rue Henri Huchard, 75018 Paris, France
P Ravaud, professor of epidemiology
INSERM U738; Groupe Hospitalier Bichat-Claude Bernard (Assistance Publique—Hôpitaux de Paris); Faculté Xavier Bichat (Université Paris 7), 46 rue Henri Huchard, 75018 Paris, France
B Giraudeau, ssistant professor of biostatistics

Editor—The meta-analysis by Bjordal et al of randomised controlled trials assessing the efficacy of non-steroidal anti-inflammatory drugs (NSAIDs) in osteoarthritic knee pain is technically well done, but the authors' conclusions are misleading.1 They assert that the mean change in pain, as measured on a visual analogue scale, over placebo was 10.1 mm (95% confidence interval 7.4 to 12.8) and claim a non-relevant difference, since the minimal clinically perceptible difference was 9.7 mm.2

They mixed up the interpretations at group and individual levels. Indeed, 9.7 is the perceptible difference at the individual level (a change lower than 9.7 mm would not be perceived by the patient). However, the 10.1 mm estimate assessed by Bjordal et al makes sense only at the group level.

To understand this multilevel interpretation better, we performed some calculations, classifying patients as improved or not if they achieved a change greater than 10.1 mm. Thus, hypothesising that in the placebo group, the change equals 5+20 mm, 40.1% of the placebo group patients would show improvement (under the reasonable assumption of a normal distribution of change in pain on a visual analogue scale). A treatment effect of 10.1 mm leads to a mean change of 15.1 mm in the experimental group, and 60.1% of patients would therefore show improvement. The number of patients needed to treat is then estimated at 5.0. In the same way, if the difference in the placebo group was 20±20 mm, 69.2% of patients would show improvement in this group and 84.3% in the experimental group, and the number needed to treat would then be 6.6. These examples show that using the minimal clinically perceptible difference to interpret changes at the group level is inadequate: a small variation at the group level does not mean no clinically relevant change in the individuals of the group.

Notes

Competing interests: None declared.

References

1. Bjordal JM, Ljunggren AE, Klovning A, Slordal L. Non-steroidal anti-inflammatory drugs, including cyclooxygenase-2 inhibitors, in osteoarthritic knee pain: meta-analysis of randomised placebo controlled trials. BMJ 2004;329: 1317. (4 December.) [PMC free article] [PubMed]
2. Ehrich EW, Davies GM, Watson DJ, Bolognese JA, Seidenberg BC, Bellamy N. Minimal perceptible clinical improvement with the western Ontario and McMaster universities osteoarthritis index questionnaire and global assessments in patients with osteoarthritis. J Rheumatol 2000;27: 2635-41. [PubMed]

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