What is number needed to treat (NNT)?

Researchers tested the effectiveness of supplements during pregnancy on pre-eclampsia in a high risk population. A three arm, randomised, blinded, placebo controlled trial design was used. Intervention was two medical food bars a day containing the supplements L-arginine and antioxidant vitamins, antioxidant vitamins alone, or placebo.1

Participants were pregnant women with a previous pregnancy complicated by pre-eclampsia, or pre-eclampsia in a first degree relative, and deemed to be at increased risk of recurrence of the disease. Women were studied from week 14-32 of gestation and followed until delivery. In total, 228 women were allocated to L-arginine and antioxidant vitamins, 222 to antioxidant vitamins alone, and 222 to placebo.

The primary outcome was development of pre-eclampsia or eclampsia. Antioxidant vitamins alone showed an observed benefit, but this effect was not significant compared with placebo. The proportion of women with pre-eclampsia or eclampsia was reduced significantly in the L-arginine plus antioxidant vitamins treatment group compared with placebo (absolute risk reduction 0.17 (95% confidence interval 0.12 to 0.21). The number needed to treat (NNT) was 5.73 (4.0 to 10.0). The researchers concluded that supplementation during pregnancy with a medical food containing L-arginine and antioxidant vitamins reduced the incidence of pre-eclampsia and eclampsia in a population at high risk of the condition.

Which of the following statements, if any, are true?

• a) The number needed to treat is a measure of benefit of the supplements compared with …