3.3 SENSITIVITY & SPECIFICITY EXERCISE (50 pts.) 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 in the recording and PPT, E.g., a + c = prevalence. Remember, be sensitive to the ill. May the Lord help you to enjoy the challenge! Evaluating Screening Tests.pdf Read the article, Evaluating Screening Tests. Then proceed to answer the scenario questions. 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 in the Week 3 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. You may calculate by hand or use the online calculator at https://www.medcalc.org/calc/diagnostic_test.php or You may calculate by hand or use the online calculator: http://vassarstats.net/clin1.html (Condition Absent or Present is determined by symptoms.) (1 point per box) /9 points Positive tests Sensitivity of 100% Ills = a / (a + c) (a) True + Sensitivity 22.9% Negative tests (c) False -, Type II Total tests (a+c) Prevalence of Ills Specificity of 100% wells = d / (b + d) Total people (b) False +, Type I (a+b) PPV (d) True - Specificity 99.8% Calculate as indicated 10. The percentage of false positives among all those without the condition (the Type I Error Rate, or 1 - specificity), or b / (b + d) 11. The percentage of false negatives among all those with the condition (the Type II Error Rate, or 1 - sensitivity), or c / (a + c) (b+d) Wells (c+d) NPV 10,000 Population Answer (2 pts. each) Points /2 /2 12. The predictive value of a positive (PPV) test, or = a / (a + b) 13. The predictive value of a negative (NPV) test, or = d / (c + d) 14. Based on the calculations above, how many false positives and negatives will occur if 100,000 people are screened? /2 /2 /2 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%. Questions 15-23: Using the information in the Week 3 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. You may calculate by hand or use the online calculator at https://www.medcalc.org/calc/diagnostic_test.php or You may calculate by hand or use the online calculator: http://vassarstats.net/clin1.html (Condition Absent or Present is determined by symptoms.) (1 point per box) /9 points Sensitivity of 100% Ills = a / (a + c) Positive tests (A) True + Sensitivity 44.3% Negative tests Total tests (C) False -, Type II (A+C) Prevalence of Ill Specificity of 100% wells = d / (b + d) (B) False +, Type I (D) True - Specificity 99.0% (B+D) Wells Calculate as indicated 24. The percentage of false positives among all those without the condition (the Type I Error Rate, or 1 - specificity), or b / (b + d) 25. The percentage of false negatives among all those with the condition (the Type II Error Rate, or 1 - sensitivity), or c / (a + c) 26. The predictive value of a positive (PPV) test, or = a / (a + b) 27. The predictive value of a negative (NPV) test, or d / (c + d) 28. Based on the calculations above, how Total people (A+B) PPV (C+D) NPV 10,000 Population Answer (2 pts. each) Points /2 /2 /2 /2 /2 many false positives (b x 10) and negatives (c x 10) will occur if 100,000 people are screened? Select a laboratory test and describe its: Answer (6 points each) 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. 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. (6 pts.) Points /6 /6