# Positive Predictive Value Calculator

Instructions: This Positive Predictive Value Calculator computes the positive predictive value (PPV) of a test, showing all the steps. In order to do so, please fill up the 2x2 table below with the information about disease presence and absence, and screening test status:

Disease Present
Disease Absent
Test +
Test -

The Positive Predictive Value a test (PPV) corresponds to the probability that a patient has the disease, given that the patient tests positive.

### How do you compute the PPV a test?

Check out the following 2x2 crosstabulation about the results of the screening test and presence of the disease:

Disease Present
Disease Absent
Test +
$$TP$$
$$FP$$
Test -
$$FN$$
$$TN$$

• The value $$TP$$ corresponds to the number of true positive cases, which is when the test shows positive for patients with the condition.

• The value $$FP$$ corresponds to the number of false positive cases, which is when the test shows positive for patients who don't have the condition.

• The value $$FN$$ corresponds to the number of false negative cases, which is when the test shows negative for patients with the condition.

• The value $$TN$$ corresponds to the number of true negative cases, which is when the test shows negative for patients who don't have the condition.

Since $$TP + FP$$ is the total number of those who test positive, the positive predictive value of the test is

$\text{PPV} = \displaystyle \frac{TP}{TP + FP}$

### What are other related metrics to positive predictive value?

When working with the characteristics of a test, you probably are going to be interested in knowing about the specificity of the test, the sensitivity of the test, as well as the negative predictive value (NPV).

### How to calculate sensitivity and specificity in excel

You need to follow the same calculation formula used above to compute the sensitivity and specificity of a test in Excel. First, you need to put the elements in the crosstabulation form specified above.

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