Test Sensitivity & Specificity

Disease       Disease  
Present Absent Total Present Absent Total
Test Pos Test Pos a: (True + ve) b: (False + ve) a + b
Neg Neg c: (False - ve) d: (False - ve) c + d
  Total   Total a + c b + d n
 
Test sensitivity: The ability of the TEST to correctly identify those with the disease (true +ve rate): (a/(a+c).
Test specificity: The ability of the TEST to correctly identify those without the disease (true -ve rate): (d/(b+d).