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About Diagnostic Test Sensitivity and Specificity
Diagnostic Tests can vary widely in how trustworthy the test results are. Laboratory testing facilities typically publish how reliable their protocols are. This Wiki page explains the terms used to describe the Accuracy for specific Laboratory Tests.
Sensitivity
The Sensitivity of a Diagnostic Test is the Probability that the test will Report a Positive Test Result when a person should obtain a Positive Test Result = aka. TRUE POSITIVE
High Sensitivity = Good Diagnostic Test
For example, I have Crohn's Disease. A Highly Sensitive Test for Crohn's Disease will give me a Positive Test Result showing I have Crohn's Disease.
 Test Result = You HAVE Crohn's  Test Result = You DO NOT have Crohn's 
I HAVE Crohn's Disease  Test Accurately Reports  You HAVE Crohn's  Test misreports you have Crohn's 
I DO NOT have Crohn's Disease  Test misreports you have Crohn's  Test Accurately Reports you do not have Crohn's 
Specificity
The Specificity of a Diagnostic Test is the Probability that the test will Report a Negative Test Result when a person should obtain a Negative Test Result = aka. TRUE NEGATIVE
High Specificity = Good Diagnostic Test
For example, I do NOT have Ulcerative Colitis. A Highly Specific Test for Ulcerative Colitis will give me a Negative Test Result showing I do not have Ulcerative Colitis.
 Test Result = You HAVE Ulcerative Colitis  Test Result = You DO NOT have Ulcerative Colitis 
I HAVE Ulcerative Colitis  Test Accurately Reports  You HAVE Ulcerative Colitis  Test misreports you have Ulcerative Colitis 
I DO NOT have Ulcerative Colitis  Test misreports you have Ulcerative Colitis  Test Accurately Reports you do not have Ulcerative Colitis 
False Positive
A False Positive Diagnostic Test Result Reports a Positive Test Result when a person should obtain a Negative Test Result.
High False Positive Rate = Bad Diagnostic Test
For example, I do NOT have Behcet's Disease. A False Positive Test Result will give me a Positive Result showing I have Behcet's Disease.
 Test Result = You HAVE Behcet's  Test Result = You DO NOT have Behcet's 
I HAVE Behcet's Disease  Test Accurately Reports  You HAVE Behcet's  Test misreports you do not have Behcet's 
I DO NOT have Behcet's Disease  Test Misreports you HAVE Behcet's Disease  Test Accurately reports you do not have Behcet's 
False Negative
A False Negative Diagnostic Test Result Reports a Negative Test Result when a person should obtain a Positive Result.
High False Negative Rate = Bad Diagnostic Test
For example, I HAVE Clostridium difficile (C. diff) Infection. A False Negative Test Result will give me a Negative Test Result when I actually do have a Clostridium difficile Infection.
 Test Result = You HAVE Clostridium difficile  Test Result = You DO NOT have Clostridium difficile 
I HAVE C. difficile  Test Accurately Reports  You HAVE C. difficile  Test misreports you do NOT have C. difficile 
I DO NOT have C. difficile  Test Misreports you HAVE C. difficile  Test Accurately reports you do not have C. difficile 
Positive Predictive Value (PPV)
The Positive Predictive Value (PPV) of a Test takes into account how many people in a population have the disease or condition in question (Disease Prevalence). Whereas Sensitivity and Specificity are a function of how good a particular Test is, the PPV goes a step further and looks at the probability a person has the disease.
PPV = (True Positive) / [(True Positive)+(False Positive)] X 100
Negative Predictive Value (NPV)
The Negative Predictive Value (NPV) of a Test takes into account how many people in a population have the disease or condition in question (Disease Prevalence). Whereas Sensitivity and Specificity are a function of how good a particular Test is, the NPV goes a step further and looks at the population based probability a person does NOT have the disease.
NPV = (True Negative) / [(True Negative)+(False Negative)] X 100
References
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