FRCS (ORL-HNS) · SECTION 1 SBA

Number needed to treat

A trial reports postoperative haemorrhage in 10% of controls and 6% with an intervention. What is the number needed to treat to prevent one haemorrhage?

CHOOSE ONE ANSWER

Best of five

  1. A. 4
  2. B. 10
  3. C. 25
  4. D. 16.7
  5. E. 40
ANSWER AND REASONING

C. 25

Absolute risk reduction is 0.10 minus 0.06 = 0.04; NNT is 1/0.04 = 25.

Why every option is right or wrong

A. 4: This confuses the four-percentage-point absolute reduction with NNT.

B. 10: This is the control event percentage, not the reciprocal absolute reduction.

C. 25: Absolute risk reduction is 0.10 minus 0.06 = 0.04; NNT is 1/0.04 = 25.

D. 16.7: This is approximately the reciprocal of the treated event rate.

E. 40: This incorrectly treats the absolute reduction as 2.5%.

What if the scenario changed?

If the treated event rate were 8%, the absolute reduction would be 2% and the NNT would be 50.

EDUCATIONAL USE

Independent Clinora preparation material; not official JCIE content, an accredited programme or a substitute for local policy and specialist judgement.

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