Page 93 - Radiology Book

ePidemiology test window
Positive Predictive Value (PPV)
a a+b
Negative Predictive Value (NPV)
d
91
Disease Window
+ -
a
TP
b
FP
c
FN
d
TN
Test + Window -
The standard 2 × 2 table comparing the test results (Test Window) and the true disease status (Disease Window). Memorize this table!
TP (a): .........................True Positive FP (b): .........................False Positive FN (c): .........................False Negative TN (d): .........................True Negative
The Test Window (blue arrow) is used to calculate the PPV and NPV. The Disease Window (red arrow) is used to calculate the sensitivity and speci city.
Evidence-Based Medicine De nitions
· Prevalence: number of people who have a disease in a de ned population at a given point in time.
· Prevalence rate = (a+c)/(a+b+c+d)
· Sensitivity: the ability of a test to detect a disease (ie, the proportion of infected patients that test positive). If a test is not sensitive, it will fail to detect disease when it is present (false negatives).
· Sensitivity = a/(a+c) or TP/(TP+FN)
· Speci city: the ability of a test to accurately indicate when
no disease is present (ie, the proportion of noninfected patients that test negative for the disease). If a test is not speci c, it will falsely indicate the presence of disease in noninfected patients (false positives).
· Spec city = d/(b+d) or TN/(TN+FP)
Sensitivity Speci city
a d c+d a+c b+d
EpidEmiology