Ideally a new test should have both high sensitivity and high specificity; that, however, can often not be achieved. Depending on the purpose of the test a high sensitivity of a test will be more important than a high specificity or vice-versa.

For example:

• Eradication of a disease: we absolutely want to avoid having false negative animals and miss affected animals.
  >> We will choose a very sensitive test.
• If a positive test is associated with lengthy and costly re-testing or culling of infected animals we may want to avoid having false positive patients.
  >> We will choose a very specific test.
• In testing blood donors for AIDS (HIV) you need a fast and highly sensitive test (since negative blood samples will be given to patients) AND a highly specific test (since you don’t want to falsely declare donors as HIV positive if they are not truly infected).
  >> Such high combined SE and SP often can only be achieved through a test system (combination of tests).