NAPLEX Question of the Week: NNT

An important biostatistical calculation is the subject of our question of the week!
NAPLEX Question of the Week: NNT

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NAPLEX Question of the Week 

A study was performed to evaluate the efficacy of oteseconazole vs. fluconazole for the treatment of recurrent vulvovaginal candidiasis (VVC). For the primary efficacy endpoint, oteseconazole was superior to fluconazole. Post randomization through week 50 demonstrated the following:

Oteseconazole group: 5.1% of participants had a recurrence of a VVC episode

Fluconazole group:  42.2% of participants had a recurrence of a VVC episode

P value= <0.001

What is the number needed to treat to prevent one recurrence of VVC over a 50 week period? 

Fill in the blank: ________


The number needed to treat (NNT) is the number of patients who need to be treated for a certain period of time (usually the study duration) in order for one patient to benefit. The formula for calculating this is: 

NNT = 1/[(% risk in control group)-(% risk in treatment group)] OR 1/ARR

ARR is absolute risk reduction, ARR= (% risk in control group)-(%risk in treatment group)

So in this case:

NNT= 1/(0.422-0.051)=2.69→ round up to 3

Correct Answer: Number needed to treat= 3 

Another point to consider when calculating the number needed to treat is to always round up to the nearest whole number. This is because there cannot be a fraction of a patient. In this question the NNT came out to 2.69 so it is rounded up to 3; however, if the answer came out to be 2.3 the NNT would also be 3 due to always rounding up. This is an important concept when looking at research and determining how many patients benefit from a treatment. This could help determine whether a treatment is right for a particular patient based on the risk/benefit ratio.

Another important component to the risk/benefit ratio is the number needed to harm (NNH). This occurs when an intervention has worse outcomes than the control, and addresses the number of patients who need to be treated for a certain time period in order for one patient to be harmed. The NNH can also be used when a particular treatment may have better efficacy but also greater toxicity, such as prasugrel vs. clopidogrel when treating acute coronary syndromes.  The NNT can be used to determine cardiovascular benefit of using prasugrel vs. clopidogrel, but the NNH can be used to determine increased risk of bleeding when using prasugrel vs. clopidogrel. When calculating NNH it is always important to round down to the nearest whole number. 

NAPLEX Core Competencies covered:

Area 1: 1.7 (Evidence-based literature or studies using primary, secondary, and tertiary references)

Area 4: 4.8 (Biostatistics, epidemiological, or pharmacoeconomic measures)

Keep up the studying! You can do this!

Dr. B

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