Comprehensive and detailed notes on chapter 4; sensitivity and specificity of screening and diagnostic tests for Hth 450.
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SENSITIVITY AND SPECIFICITY OF SCREENING
AND DIAGNOSTIC TESTS
Objectives
Discuss why diagnostic and screening tests are important in epidemiology and clinical practice
Define and discuss sensitivity and specificity
Discuss how screening levels for continuous variables affect sensitivity and specificity
Diagnostic Screening Tests
To understand how a disease is transmitted and develops and to provide appropriate and
effective health care:
o Necessary to distinguish between people in the population WHO HAVE the disease and
those WHO DO NOT
Thus, the quality of screening and diagnostic tests are a critical issue
Types of diagnostic/screening tests -
o Physical exam, chest x-ray, EEG, blood/urine assay
Two important questions:
o How good is the test in separating populations of people with and without the disease in
question?
o How do we assess the quality of newly available screening and diagnostic tests in order
to make reasonable decisions about their use and interpretation?
Validity of Screening Tests
Validity
o The ability to distinguish between who has the disease and who does not
Validity of screening/diagnostic tests have two components:
o Sensitivity
The ability of the test to correctly identify those who HAVE the disease
o Specificity
The ability of the test to correctly identify those WHO DO NOT have the disease
Tests with Dichotomous Results (Positive or Negative)
To calculate the sensitivity and specificity:
o We must know who 'really' has the disease and who does not from a source other than
the test we are using
We are comparing our test results to an external source of 'truth' regarding the disease status of
each individual in the population
Sometimes this 'truth' may be the result:
o Of another 'gold standard' tests
o Of a more definitive, more invasive test (e.g., tissue biopsy)
Suppose we have a hypothetical population of 1000 people
o Of whom 100 have a certain disease
A test is available that can yield either positive or negative results
o We want to use this test to try and distinguish persons who have the disease from those
that do not
How good was the test in identifying people with the disease?
o F
Sensitivity is calculated by finding the proportion of the diseased people who were correctly
identified as positive by the test
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