Which formula correctly defines PPV?

Which formula correctly defines PPV?

• A. TN / (TN + FN)
• B. TP / (TP + FP)
• C. TP / (TP + FN)
• D. FN / (FN + FP)

Answer: (B)

Positive Predictive Value tells you, among all the people the test marks as positive, how many truly have the condition. This is captured by comparing true positives to all positives—the test’s positives that are correct and those that are incorrect. So the correct formula is true positives divided by the sum of true positives and false positives. Think of it this way: if 100 people test positive, and 80 actually have the disease while 20 do not, PPV is 80/100 = 0.8. The numerator counts only the correct positives, and the denominator counts every positive result the test produced. The other formulas point to different performance measures: using false negatives in the numerator relates to sensitivity (the ability to detect actual positives), and using true negatives with false negatives relates to negative predictive value (the chance that a negative test is truly negative). The leftover option does not reflect PPV.

Positive Predictive Value tells you, among all the people the test marks as positive, how many truly have the condition. This is captured by comparing true positives to all positives—the test’s positives that are correct and those that are incorrect. So the correct formula is true positives divided by the sum of true positives and false positives.

Think of it this way: if 100 people test positive, and 80 actually have the disease while 20 do not, PPV is 80/100 = 0.8. The numerator counts only the correct positives, and the denominator counts every positive result the test produced.

The other formulas point to different performance measures: using false negatives in the numerator relates to sensitivity (the ability to detect actual positives), and using true negatives with false negatives relates to negative predictive value (the chance that a negative test is truly negative). The leftover option does not reflect PPV.