Sensitivity vs. Specificity of a test

[qaiser]

From clinical point of view, it is very important to understand the concept of sensitivity and specificity of a test.
Sensitivity: Suppose 10 subjects were screened for a test. In reality all 10 did have the disease but test came positive only for 8 people. This test failed to catch 2 people who had the disease. Test showed false negative tests for those two persons. This test is only 80 % sensitive.
Sensitivity is calculated by the following formula:
True positives/True positives+False negative
An ideal test will have a high sensitivity meaning it will catch all people with a disease. If a test has a low sensitivity, it wouldn't be reliable because as a clinician based in false negative, you will tell your patient that he/she does not have the disease while in reality he/she is diseased. It is like letting loose a criminal.
Specificity: Suppose 10 persons were tested for a disease and all 10 people came positive for that disease. In reality, only 8 people had the disease. The two people who came positive do not have the disease in reality. They have some other condition that gives a false positive.
If a test has a low specificity, it is not relaiable because it will give false positive results.
The formula for specificity is:
True negative/true negative+False positive
An example of a test that is not very specific is VDRL and RPR (syphilis diagnostic tests). These tests show positive results for SLE (systemic lupus erythematosus).
Any comments are welcomed.
Good luck in your studies.

[stephew]

know the different beween this and positive and negative predictive value. they love that and its very usefyul to use.

[qaiser]

Stephew,
Thanks for your input. Yes perdictive value is an important topic indeed. My next post is about predictive value. By the way what was those lines about warning below your reply? I did not quite get that. Thanks again for your reply. Good luck in your studies.

[Sree Cheruku]

MANPANTS! duh.. don't you own a pair? They help you not whine when you get a warning.

[redeye]

Stephew,
Thanks for your input. Yes perdictive value is an important topic indeed. My next post is about predictive value. By the way what was those lines about warning below your reply? I did not quite get that. Thanks again for your reply. Good luck in your studies.

Yes, everyone always seems to emphasize sensitivity and specificity when discussing tests, but prevelence to me is the most important and most ignored.

Take a test that is both 99% sensitivity and 99% selectivity (sounds great)

If the prevelence for the disease you are testing for is 1/10,000 then you will have a ~1% probability that a positive result is an actual positive (99% of the patients will be false positive). So the predictive value is only 1%.

[qaiser]

Hi Redeye!
Thanks for your input. Good luck in your studies.

[redeye]

Hi Redeye!
Thanks for your input. Good luck in your studies.

Thank you. Good luck!!!