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Sensitivity & Specificity

Jessica Mason, MD and Mel Herbert, MD MBBS FAAEM
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Crunch Time EM - EBM - Written Summary 607 KB - PDF

Sensitivity & Specificity

Jessica Mason MD and Mel Herbert MD

  • Sensitivity
    • The proportion of patients with the disease who test positive (see formula below)
      • How good is the test at picking up anyone who might have the disease
      • E.g. using troponin to test for ACS
        • Newer troponins are even more sensitive than prior generations
        • But this does not mean that everyone with a slightly elevated troponin is having an MI
  • Specificity
    • Proportion of patients without the disease who test negative
      • How good is the test at confirming the diagnosis?
      • E.g. using troponin to test for ACS
        • Many patients who have heart failure, renal disease, or some other cardiac issue such as myocarditis can have an elevated troponin
        • Troponin is not very specific to ACS -- it still needs to be used in the right clinical setting
  • Calculating sensitivity and specificity
    • Set up a 2 x 2 table as shown in table 1
      • Sensitivity = true positive / (true positive + false negative)
      • Specificity = true negative / (true negative + false positive)
  • Example with made-up data:
    • There are 100 patients, 10 have ACS, of those 10 only 9 have a positive troponin
    • 90 Patients do not have ACS, but 30 have a positive troponin
    • See table 2 to see the example 2x2 table
      • Sensitivity = 9 / (9+1) = 90%
      • Specificity = 60 / (60+30) = 67%
    • In this example, troponin is fairly sensitive but not very specific

 

 

Image of Sensitivity & Specificity 2x2 TableSensitivity & Specificity 2x2 Table

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