sensitivity and specificity

FREE subscriptions for doctors and students... click here
You have 3 more open access pages.

These are statistical terms that are often confused. These terms can be illustrated by way of an example:

At a particular hospital a MIBG scanner is used to detect phaeochromocytomas. A positive scan was reported in 38 cases. An audit showed:

  • true positive (TP) 10, true negative (TN) 25,
  • false positive (FP) 0, false negative (FN) 3

where:

  • patients with phaeochromocytoma TP (person with a phaeochromocytoma and positive test)
  • patients with phaeochromocytoma FN (person with a phaeochromocytoma and negative test)
  • patients without phaeochromocytoma FP (person without a phaeochromocytoma with a positive test)
  • patients without a phaeochromocytoma TN (person without a phaeochromoytoma with a negative test)

phaeochromocytoma present phaeochromyctoma absent

test result positive

TP = 10

test result positive

FP = 0

test result negative

FN = 3

test result negative

TN = 25

  • sensitivity = 100xTP /(TP+FN) = 100 x 10(10 + 3) = 77%
    • i.e. proportion of people with a phaeochromocytoma who are corectly identified by the screening test = 77%

  • specificity = 100xTN /(TN+FP) = 100 x 25 /(25+0) = 100%
    • i.e. proportion of people without phaeochromocytoma who have had a negative test result = 100%

  • positive predictive value = 100xTP/(TP+FP) = 100 x 10/(10+0) = 100%

i.e. the proportion of screening tests that are correct.

Last reviewed 10/2021

Links: