Positive Predictive Value - Worked Example

Worked Example

Suppose the fecal occult blood (FOB) screen test is used in 2030 people to look for bowel cancer:

Patients with bowel cancer
(as confirmed on endoscopy)
Condition Positive Condition Negative
Fecal
Occult
Blood
Screen
Test
Outcome		 Test
Outcome
Positive		 True Positive
(TP) = 20		 False Positive
(FP) = 180		 Positive predictive value = TP / (TP + FP)
= 20 / (20 + 180)
= 10%
Test
Outcome
Negative		 False Negative
(FN) = 10		 True Negative
(TN) = 1820		 Negative predictive value = TN / (FN + TN)
= 1820 / (10 + 1820)
≈ 99.5%
Sensitivity = TP / (TP + FN)
= 20 / (20 + 10)
≈ 67% 		Specificity = TN / (FP + TN)
= 1820 / (180 + 1820)
= 91%

The small positive predictive value (PPV = 10%) indicates that many of the positive results from this testing procedure are false positives. Thus it will be necessary to follow up any positive result with a more reliable test to obtain a more accurate assessment as to whether cancer is present. Nevertheless, such a test may be useful if it is inexpensive and convenient. The strength of the FOB screen test is instead in its negative predictive value - which, if negative for an individual, gives us a high confidence that its negative result is true.

Read more about this topic:  Positive Predictive Value

Famous quotes containing the word worked:

    “...we never worked for white people in their homes. No, sir, not even once! That is one of the accomplishments in my life of which I am the most proud, yes, sir!”
    —Annie Elizabeth Delany (b. 1891)

    “Fasten your hair with a golden pin,
    And bind up every wandering tress;
    I bade my heart build these poor rhymes:
    It worked at them, day out, day in,
    Building a sorrowful loveliness
    Out of the battles of old times.”
    —William Butler Yeats (1865–1939)