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  #1  
Old 03-12-2015
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Default sensitivity and specificity

hello.
I am very confused by the two terms

sensitivity is the test to detect people WITH disease. but then FA continues to say that 100% sensitivity is desirable to RULE OUT disease, and indicates low false negative rate.

Shouldn't I RULE IN disease if I my test is 100% sensitive and not rule it out?!
Specificity is the test to detect people WITHOUT disease. but then FA continues to say that 100% specificity is desirable to RULE IN disease, and indicates low false positive rate.

Shouldn't I RULE OUT disease if I my test is 100% specific and not rule it in?!

I am completely confused, any help would be greatly appreciated.
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  #2  
Old 03-13-2015
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I agree this is a confusing concept but just remember the mnemonics SNout and SPin. A test with high sensitivity will detect most cases with the disease. So if a person gets a negative in a test with high sensitivity we can be sure that he will not be having the disease and hence tests with high sensitivities are good rule out tests. Screening tests usually have high sensitivities so that we dont miss any pts with the disease and so that we are sure that ppl who are negative dont have the disease.Tests with high sensitivities will however have a high false positive rate(i.e that they may identify some non diseased ppl with the diesease) so in order to deal with this we do a confirmatory test. A confirmatory test usually has high specificity and so makes sure that anyone who tests positive will actually have the disease and this is why it is a rule in test.
an example would be ELISA P24 which is done first to screen for AIDS. This test has high sensitivity and so it doesn't miss many pts and ppl with negative Elisa p 24 are most certainly negaitive for the disease(rule out test ).This is followed by western blot which has high specificity and so when positive makes sure that pts actually have the disease.
Hope iv'e helped and not made this more confusing
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  #3  
Old 03-13-2015
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sensitivity= it responds to even slightest stimuli
so there is no possibility of leaving out bacteria(for e.g.)
so u will never have a false negative result i.e. a result where u are sure you have not left out any of those bac you r 100% sure that if the result is negative there is no possibility of that disease. 100% u can rule out the disease.

specificity=it responds only to that specific stimuli
so when u say a test which is highly specific is positive u know definitely it is that bacteria no other bacteria will give that result so no false positive results.
100% u can rule in the disease.

i hope that
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Old 03-13-2015
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Makes much more sense now! thanks to both of you!!
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Old 02-23-2016
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I'll try to add to the above comments to help increase clarity.

Sensitivity
A test that is 100% sensitive means that no one with the condition will test negative. Everyone with the condition will test positive, as well as some people without the condition. Thus, a negative test means that the condition can be effectively ruled out.

Specificity
A test that is 100% specific means that no one without the condition will test positive. Everyone without the condition will test negative, as well as some people with the condition. Thus, a positive test means that the condition can be effectively ruled in.
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  #6  
Old 04-29-2016
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Default Tradeoff between sensitivity and specificity

heres a lecture that i put on youtube that might help clarify the issue.... Trade-Off: Sensitivity vs. Specificity.

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