Number needed to treat vs Number needed to harm
How to tell Number needed to treat and Number needed to harm apart when a board question makes them look alike.
NNT: patients treated to prevent one bad outcome (1 / ARR), lower is better; NNH: patients treated to cause one harm (1 / ARI), higher is better.
Side by side
Number needed to treat is the number of patients who must receive therapy for one additional beneficial outcome, calculated as 1 divided by absolute risk reduction.
Number needed to harm is the number exposed for one additional adverse outcome, calculated as 1 divided by absolute risk increase.
Number needed to treat is the number of patients who must receive therapy for one additional beneficial outcome, calculated as 1 divided by absolute risk reduction.
Number needed to harm is the number exposed for one additional adverse outcome, calculated as 1 divided by absolute risk increase.
NNT = 1 / absolute risk reduction; lower is better.
NNH = 1 / absolute risk increase; higher is safer.
The trap
NNT uses absolute risk reduction for benefit; NNH uses absolute risk increase for harm.
The repair move
Before naming the answer, force the stem through the separator: NNT uses absolute risk reduction for benefit; NNH uses absolute risk increase for harm.
Try a free question
CoreA clinical trial of statin therapy in primary prevention shows that over 5 years, statin reduces MI from 5% to 3.5%. What is the NUMBER NEEDED TO TREAT (NNT) to prevent one MI?
Show answer and explanation
Answer: A. NNT = 1/ARR. NNT = 1 / ARR (absolute risk reduction). NNT represents the number of patients who must be treated to prevent ONE additional bad outcome compared to control. Lower NNT = more effective. NNT of 67 means treating 67 patients for 5 years prevents 1 MI. Compare to NNH (number needed to harm) for safety.
Why the other choices are tempting
- B. NNH = 1/ARI is the number needed to harm.
- C. RRR (relative risk reduction) is (1 - RR), here 30%, a different statistic.
- D. Hazard ratio is from Cox regression.
- E. Power is the statistical concept, not NNT.
Flashcards
Tap a card to flip it.
What clue supports Number needed to treat?
Choose Number needed to treat when the stem gives this pattern: Number needed to treat is the number of patients who must receive therapy for one additional beneficial outcome, calculated as 1 divided by absolute risk reduction.
What clue supports Number needed to harm?
Choose Number needed to harm when the stem gives this contrasting pattern: Number needed to harm is the number exposed for one additional adverse outcome, calculated as 1 divided by absolute risk increase.
Quick answers
What is the fastest way to tell Number needed to treat from Number needed to harm?
NNT: patients treated to prevent one bad outcome (1 / ARR), lower is better; NNH: patients treated to cause one harm (1 / ARI), higher is better.
What is the key clue for Number needed to treat?
Number needed to treat is the number of patients who must receive therapy for one additional beneficial outcome, calculated as 1 divided by absolute risk reduction.
What is the key clue for Number needed to harm?
Number needed to harm is the number exposed for one additional adverse outcome, calculated as 1 divided by absolute risk increase.
What is the common trap with Number needed to treat vs Number needed to harm?
NNT uses absolute risk reduction for benefit; NNH uses absolute risk increase for harm.