sensitivity specificity epidemiology
sensitivity & specificity
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Similar to converting Fahrenheit to Celsius (E.g., 98.6o F – 32 x 5 / 9 = 37o C) and back (E.g., 37o C x 9 / 5 + 32 = 98.6o F), these problems require you to work backwards and forward from the data given.Also, all the clues and formulas you need are on PPT, E.g., a + c / total pop = prevalence.Remember, be sensitive to the ill. |
Read the Evaluating Screening Tests. Then proceed to answer the scenario questions. See attached file |
Scenario 1: The prevalence (a + c in the table below) of previously undetected diabetes in a population to be screened is approximately 1.5% and it is assumed that 10,000 persons will be screened.The screening test will measure blood serum sugar content.A value of 180 mg/dl or higher is considered positive.The sensitivity (a) and the specificity (d) associated with this screening are 22.9% and 99.8% respectively.
Questions 1-9: Using the information on the PPT set up a two by two table with the appropriate numbers in each cell of the table.Round to the nearest whole number, but only after you have completed all the calculations down through.(Condition Absent or Present is determined by symptoms.) (1 point per box)
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Ills |
Wells |
Total people |
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Positive tests |
(A or True +)
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(B or False +)
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(A+B or all positive tests)
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Negative tests |
(C or False – )
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(D or True -)
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(C+D or all negative tests)
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Total tests |
(A+C or all ills)
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(B+D or all wells)
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10,000 Total Population |
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Calculate as indicated |
Answer |
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10. The percentage of false positives among all those without the condition (the Type I Error Rate, or 1 – specificity), or b / (b + d) |
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11.The percentage of false negatives among all those with the condition (the Type II Error Rate, or 1 – sensitivity), or c / (a + c) |
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12.The predictive value of a positive (PPV) test, or = a / (a + b) |
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13.The predictive value of a negative (NPV) test, or = d / (c + d) |
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14. Based on the calculations above, how many false positives and negatives will occur if 100,000 people are screened? |
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Scenario 2: To observe the effect of increasing sensitivity, assume a blood sugar screening level of 130 mg/dl, with (A) sensitivity of 44.3% and (D) specificity of 99.0%. Use the values for a + c from the table below to calculate prevalence.
Questions 15-23: Using the information in thePPT, set up a two by two table with the appropriate numbers in each cell of the table.Round to the nearest whole number, but only after you have completed all the calculations down through.(Condition Absent or Present is determined by symptoms.) (1 point per box)
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Ills |
Wells |
Total people |
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Positive tests |
(A or True +)
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(B or False +)
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(A+B or all positive tests)
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Negative tests |
(C or False -)
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(D or True -)
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(C+D or all negative tests)
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Total tests |
(A+C or all ills) |
(B+D or all wells)
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10,000 Total Population |
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Calculate as indicated |
Answer |
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24.The percentage of false positives among all those without the condition (the Type I Error Rate, or 1 – specificity), or b / (b + d) |
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25. The percentage of false negatives among all those with the condition (the Type II Error Rate, or 1 – sensitivity), or c / (a + c) |
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26.The predictive value of a positive (PPV) test, or = a / (a + b) |
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27. The predictive value of a negative (NPV) test, or d / (c + d) |
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28.Based on the calculations above, how many false positives (b x 10) and negatives (c x 10) will occur if 100,000 people are screened? |
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Select a laboratory test and describe its: |
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29. Explain the clinical significance of a diagnostic test’s sensitivity and specificity. Be specific in your explanation by using a diagnostic test as an example. |
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30.Compare the findings of Scenario 1 and 2.If you were the director for the diabetes screening program would you prefer to screen at 130 mg or 180 mg percent? Explain why. Resources:
Textbook: Macha, K., & McDonough, J. P. (2012). Epidemiology for advanced nursing practice. Sudbury, MA: Jones and Bartlett Learning Chapters 2 and 6. |
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