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Difficulty: MediumMedical Literature and Drug Advertisement Interpretation

An internist is discussing a newly approved direct oral anticoagulant with a 72-year-old male patient who has non-valvular atrial fibrillation. The patient brings in a direct-to-consumer print advertisement he found in a magazine. The headline reads, "Cut your risk of a stroke by a third!"

To provide accurate counseling, the internist reads the detailed prescribing information included in the advertisement's inset. The data stems from a 2-year randomized clinical trial that enrolled a total of 8,000 patients. Half of the patients received standard warfarin therapy, and the other half received the new medication. Over the 2-year follow-up period, 240 patients in the warfarin group experienced an ischemic stroke, compared to 160 patients in the new medication group.

Based on the data provided in the advertisement's inset, what is the number needed to treat (NNT) with the new medication for 2 years to prevent one additional stroke compared to warfarin?

Answer: 50 patients

Answer

50
To determine how many people must take the medication to prevent one stroke, the Number Needed to Treat (NNT) must be calculated. First, determine the Absolute Risk Reduction (ARR). The Control Event Rate (CER) with warfarin is 2404000=0.06\frac{240}{4000} = 0.06 (6%6\%). The Experimental Event Rate (EER) with the new DOAC is 1604000=0.04\frac{160}{4000} = 0.04 (4%4\%). The ARR is the difference between these rates: 0.06−0.04=0.020.06 - 0.04 = 0.02 (2%2\%). The NNT is the inverse of the ARR: 10.02=50\frac{1}{0.02} = 50. Therefore, 50 patients need to be treated with the new DOAC instead of warfarin for 2 years to prevent one additional stroke.

Step-by-Step Solution

1
Determine the number of patients in each treatment arm.
4,000 patients received warfarin, and 4,000 patients received the new medication.
The trial enrolled 8,000 patients in total, and half were assigned to each therapy.
2
Calculate the Control Event Rate (CER).
CER=2404000=0.06CER = \frac{240}{4000} = 0.06 (or 6%6\%).
CER represents the baseline risk of the outcome (stroke) in the control group over the study period.
3
Calculate the Experimental Event Rate (EER).
EER=1604000=0.04EER = \frac{160}{4000} = 0.04 (or 4%4\%).
EER represents the risk of the outcome in the group receiving the new medication.
4
Calculate the Absolute Risk Reduction (ARR).
ARR=CER−EER=0.06−0.04=0.02ARR = CER - EER = 0.06 - 0.04 = 0.02 (or 2%2\%).
ARR measures the absolute difference in risk between the standard therapy and the new therapy.
5
Calculate the Number Needed to Treat (NNT).
NNT=1ARR=10.02=50NNT = \frac{1}{ARR} = \frac{1}{0.02} = 50.
NNT indicates how many patients must receive the new treatment instead of the standard treatment to prevent one additional adverse outcome.

Key Concept

The Number Needed to Treat (NNT) is calculated as the inverse of the Absolute Risk Reduction (1/ARR1 / ARR). It represents the number of patients who need to be treated with a specific intervention to prevent one additional negative outcome.

Alternative Method

Another way to calculate NNT directly without finding event rates first is dividing the total number of patients in one group by the absolute difference in the number of events between the groups. In this scenario: NNT=4000240−160=400080=50NNT = \frac{4000}{240 - 160} = \frac{4000}{80} = 50.
Estimated Time:1m 30s
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Topics

Medical Literature and Drug Advertisement Interpretation

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