10. Screening test evaluation (sensitivity, specificity, predictive values, ROC)

Question 1

What is Sensitivity?
(true positive rate)

Answer 1

• Proportion of all people with disease who test positive, or the probability that when the disease is present, the test is positive.
• Value approaching 100% is desirable for ruling out disease and indicates a low false-negative rate.
  (Used for screening).

Question 2

How do you calculate Sensitivity ?
(true positive rate)

Answer 2

= TP / (TP + FN)
= 1 – FN rate

Question 3

What is Specificity?
(true negative rate)

Answer 3

• Proportion of all people without disease who test
  negative, or the probability that when the disease is absent, the test is negative.
• Value approaching 100% is desirable for ruling in disease and indicates a low false-positive rate.
• Used for confirmation after a positive screening test.

Question 4

How do you calculate Specificity?

Answer 4

= TN / (TN + FP)
= 1 – FP rate

Question 5

What is Positive predictive value (PPV)?

Answer 5

• Probability that a person who has a positive test result actually has the disease.
• PPV varies directly with pretest probability (baseline risk, such as prevalence of disease):
  high pretest probability → high PPV.

Question 6

How do you calculate Positive predictive value (PPV)?

Answer 6

= TP / (TP + FP)

Question 7

What is Negative predictive value (NPV)?

Answer 7

• Probability that a person with a negative test result actually does not have the disease.
• NPV varies inversely with prevalence or pretest
  probability.

Question 8

How do you calculate Negative predictive value (NPV)?

Answer 8

= TN / (TN + FN)