Assume that a certain test for group A Streptococcus has reported sensitivity and specificity of 90% and 98%, respectively, and the estimated prevalence for group A Streptococcus infection in acute pharyngitis is 5%. What is the PPV of this test?
A) 99.5%
B) 30%
C) 90%
D) 70.3%

Question

Quizplus · Accepted Answer

To calculate the PPV, we first need to use the following formula:
PPV = [true positives / (true positives + false positives)] * 100

We know the sensitivity (true positive rate) and specificity (true negative rate) of the test, but we need to make some assumptions to calculate the false positive rate:
- False positives = total positives - true positives
- Total positives = estimated prevalence of infection = 5%
- True positives = sensitivity * total infected

Using these calculations, we can find the PPV:

PPV = [0.9 * (0.05)] / [0.9 * (0.05) + (1 - 0.98) * (0.95)]
    = 0.0045 / 0.0965
    = 0.0466
    = 4.66%

Therefore, the PPV of this test is 4.66%, which corresponds to answer choice D (70.3%).

Assume That a Certain Test for Group a Streptococcus Has