A positive test result does not necessarily mean a patient has the disease; a false positive may still occur. More false positives will lower the PPV.
This refers to the probability of having a disease out of all the people who tested positive for it.
A true positive is a person who has a disease and tests positive for the disease. TP will be the numerator in the formula.
True positives (TP) will be the numerator that is divided by the calculated denominator below.
The denominator is the sum of all positive results for a given test. This will include people who do not have a disease but test positive for a disease, otherwise known as a false positive.
Add the number of true positives to the number of false positives to obtain the denominator in the equation.
Prevalence is the amount of patients who are currently affected by a disease. A common disease will have a high prevalence, while a rare disease will have a low prevalence. As the prevalence of a disease increases, the positive predictive value increases. High prevalence diseases also have a high pretest probability, and both directly correlate with a higher PPV.
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