BS - Epi/Biostat (Evaluation of diagnostic tests)
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What is the basic premise on which the evaluation of diagnostic tests is based? What are the 4 major measures of evaluation? Draw a visual defining each of them.
Uses 2x2 table comparing test results with the actual presence of disease. TP = true positive; FP = false positive; TN = true negative; FN = false negative; See p. 51 in First Aid 2014 for visual at top right corner
Which properties of a test are fixed?
Sensitivity and specificity are fixed properties of a test (vs. PPV and NPV).
What is another name for Sensitivity? What is its definition?
Sensitivity (true-positive rate); Proportion of all people with disease who test positive, or the probability that a test detects disease when disease is present
What is the equation for Sensitivity? How does it relate to false negative rate?
Sensitivity = TP / (TP + FN) = 1 - false negative rate
For what is a sensitivity value approaching 100% desirable? What does it indicate, and how is that understood using equations?
Value approaching 100% is desirable for ruling out disease and indicates a low false-negative rate; If sensitivity is 100%, TP / (TP + FN) = 1, FN = 0, and all negatives must be TNs; Think: “SN-S-OUT = highly SeNsitive test, when Negative, rules OUT disease.”
For what is a high sensitivity test used?
High sensitivity test used for screening in diseases with low prevalence
What is another name for Specificity? What is its definition?
Specificity (true-negative rate); Proportion of all people without disease who test negative, or the probability that a test indicates non-disease when disease is absent
What is the equation for Specificity? How does it relate to false positive rate?
Specificity = TN / (TN + FP) = 1 - false positive rate
For what is a specificity value approaching 100% desirable? What does it indicate, and how is that understood using equations?
Value approaching 100% is desirable for ruling in disease and indicates a low false-positive rate.; If specificity is 100%, TN / (TN + FP) = 1, FP = 0, and all positives must be TPs; Think: “SP-P-IN = highly SPecific test, when Positive rules IN disease”
For what is a high specificity test used?
High specificity test used for confirmation after a positive screening test
What defines the positive predictive value (PPV)? What is another way to state or conceptualize this?
Proportion of positive test results that are true positive; Probability that person actually has the disease given a positive test result
What is the equation for positive predictive value (PPV)?
PPV = TP / (TP + FP)
With what does PPV vary, and how so?
PPV varies directly with prevalence or pretest probability: high pretest probability => high PPV
What defines negative predictive value (NPV)? What is another way to state or conceptualize this?
Proportion of negative test results that are true negative; Probability that person actually is disease free given a negative test result
What is the equation for negative predictive value (NPV)?
NPV = TN / (FN + TN)
With what does NPV vary, and how so?
NPV varies inversely with prevalence or pretest probability: high pretest probability => low NPV
Draw a graph (x axis = test results & y axis = number of people) depicting the following possible cutoff values related to the evaluation of diagnostic tests: (A) 100% sensitivity cutoff value (B) Practical compromise between specificity and sensitivity (C) 100% specificity cutoff value.
See p. 51 in First Aid 2014 for visual at bottom of page