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Old 03-30-2011
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Stats sensitivity and specificity question? need explanation

source usmle world q bank

In clinical practice, the "CAGE" questionnaire is used to screen patients for alcoholism. When a patient replies with 2 out of 4 positive responses to the "CAGE" questions. the chance of his/her being an alcoholic. -is 100%. If this criteria is changed so that 3 out of 4 positive responses to the "CAGE" questions label the patient as alcoholic ;what is the effect on the sensitivity and specificity of this test?

A. Both sensitivity and specificity of the test will decrease
B. Both sensitivity and specificity of the test will increase
C. Sensitivity will decrease but specificity will increase
D. Sensitivity will increase but specificity will decrease
E. Both sensitivity and specificity will remain the same
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Old 03-30-2011
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the correct answer by usmle world is

C Sensitivity will decrease but specificity will increase
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Old 03-30-2011
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By changing the criteria from two positive answers to three positive answers, you'd be making the test more rigorous - less people would have a "positive CAGE questionnaire test", but it's more likely that those who do really are alcoholics (greater fraction of "positives" are "true positives", which means less are "false positives") -> increasing specificity. By making the criteria more rigorous, though, you might also miss a few people who really are alcoholics but only responded positively to two of the questions (increasing "false negatives") ->decreasing sensitivity.
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Old 03-30-2011
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its D INCREASED SENSITIVICITY AND DECRESASED SPECIFICITY.
USMLEWORLD MUST HAVE GOTTEN THAT ONE WRONG
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Old 03-30-2011
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Correct Answer Explanation of the correct Answer "C"

I always think think this way;
Highly specific means less false positives
Highly sensitive means less false negatives

So when you make it three questions out of four instead of 2/4, you have increased the cut off value. (it's just like saying hypertension is defined @ 160 mmHg instead of 140 mmHg), this means you will have less false positive results > you are more sure of identifying diseased persons > This means the test will be more specific.

Initially, 2/4 cut off > you will be labeling those with 1/4 as non-alcoholic
Now, 3/4 cut off > you will be labeling those with 1/4 and 2/4 as non-alcoholic > you have higher risk of false negatives > the test is less sensitive.
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Old 03-30-2011
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C. Sensitivity will decrease but specificity will increase

First = 50% correct --> you win
Now = 75% correct --> you win

Life is harder now
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