Medical Statistics Made Easy, Fourth Edition by Michael Harris;Gordon Taylor;

Author:Michael Harris;Gordon Taylor;
Language: eng
Format: epub
Publisher: Book Network Int'l Limited trading as NBN International (NBNi)
Published: 2020-08-25T00:00:00+00:00

This is known as the Positive Predictive Value (PPV).

Negative Predictive Value. If the test result is negative, what is the likelihood that the patient will be healthy?

Here we use: D

D + C

This is known as the Negative Predictive Value (NPV).

In a perfect test, the sensitivity, specificity, PPV and NPV would each have a value of 1. The lower the value (the nearer to zero), the less useful the test is in that respect.

EXAMPLES

Confused? Try working through an example.

Imagine a blood test for gastric cancer, tried out on 100 patients admitted with haematemesis. The actual presence or absence of gastric cancers was diagnosed from endoscopic findings and biopsy. The results are shown in Table 8.

Table 8. Two-way table for blood test for gastric cancer

Gastric cancer:

Present Absent

Blood result: Positive 20 30

Negative 5 45

Sensitivity = 20

20 + 5

= 20

25

= 0.8

If the gastric cancer is present, there is an 80% (0.8) chance of the test picking it up.

Specificity = 45

30 + 45

= 45

75

= 0.6

If there is no gastric cancer there is a 60% (0.6) chance of the test being negative – but 40% will have a false positive result.

PPV = 20

20 + 30

= 20

50

= 0.4

There is a 40% (0.4) chance, if the test is positive, that the patient actually has gastric cancer.

NPV = 45

45 + 5

= 45

50

= 0.9

There is a 90% (0.9) chance, if the test is negative, that the patient does not have gastric cancer. However, there is still a 10% chance of a false negative, i.e. that the patient really does have gastric cancer.

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