Rapid Research Review: PPV and NPV

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An Initiative of the EMRA Research Committee

Terms

Positive Predictive Value (PPV): The probability that a positive test result predicts disease
Negative Predictive Value (NPV): The probability that a negative test result predicts no disease

Disease  No Disease 
Test + A B
Test – C D

 PPV= A/(A+B)     NPV= D/(D+C)

Example: Transvaginal ultrasound (TVUS) has a 94% PPV and 100% NPV in the diagnosis of ectopic pregnancy.

What does this mean?

PPV: If a patient has a positive TVUS, there is a 94% probability that they have an ectopic pregnancy.
NPV: If a patient has a negative TVUS, there is a 100% probability that they do NOT have an ectopic pregnancy.

Extra Point: As the prevalence of a disease increases, the PPV increases. As the prevalence of a disease decreases, the NPV increases.

 

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2 Comments

  1. Love the Rapid Research Review! It’s an area that I frequently find myself going back to double check my work. That being said, this table jumped out at me when I was perusing the current issue of EMResident. The PPV and NPV equations need a small edit:

    PPV = true positive / test positive = true positive / (true positive + false positive) or A/(A+C) in this chart
    NPV = true negative / test negative = true negative / (true negative + false negative) or D/(B+D) in this chart

    The equations published in the article would give sensitivity and 1-specificity (aka false positive ratio).
    Hope this helps! Keep up the good work!

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