Week 9 - validity of diagnostic measures

Question 1

Purpose of diagnostic Accuracy studies

Answer 1

• determine if ‘new” test (index test) is accurate based on gold standard (reference test)
• true status = presence/absence of condition of interest

Question 2

design of diagnostic accuracy studies

Answer 2

• cohort studies
• participants: people at risk for condition of interest
• procedure: all participants administered index test and gold standard test
• outcomes of both tests are compared

Question 3

Validity in diagnostic design

Answer 3

• researcher taking index measurement should be blinded to result of reference test
• testers should be evaluated for reliability
• timing of index and reference test
• standards for reporting of diagnostic accuracy (STARD): checklist for essential elements of design, analysis, interpretation of diagnostic test studies

Question 4

Sensitivity:

Answer 4

how correct is test for people with disorder
- True positive/(true positive+False negative)

Question 5

Specificity

Answer 5

how correct is test for people without disorder
- = True negative/(false positive+true negative)

Question 6

Positive predictive value

Answer 6

= probability of having the disorder if test results is positive
- PPV is analogous to clinical situation - looking at the probability the person has disorder- PPV = TP/(TP+FP)

Question 7

Negative predictive value

Answer 7

• Probability of not having disorders if index test result is negative
  = TN/(TN+FN)

Question 7

What are PPV and NPV dependent on and define it

Answer 7

• prevalence: proportion of population with given disorder at given point in time
• values vary in different populations with different disorder prevalence

Question 7

Likelihood ratios

Answer 7

• Combine benefits of sensitivity and specificity into one index
• indicate how much a diagnostic test will raise of lower the pretest probability of disorder

Question 8

Pretest probability:

Answer 8

• estimation of likelihood person has disorder based on history and interview

Question 9

Sensitivity

Answer 9

= TP/(TP+FN)

Question 10

Specificity

Answer 10

TN/(TN+FP)

Question 11

Positive likelihood ratio

Answer 11

• probability that person with disorder will test positive
  = SENS/ (1-SPec)

Question 12

Negative likelihood ratio

Answer 12

• probability that person with disorder will test negative
  = (1-Sens)/Spec=FN/TN