EAB: Diagnostic Tests and Screening

Question 1

Define validity.

Answer 1

Validity = truth of result, lack of bias

Question 2

Define reliability.

Answer 2

Reliability = consistency of results

Question 3

How can you improve reliability in a test?

Answer 3

• repeatability
• reproducibility

Question 4

What is sensitivity?

Answer 4

Sensitivity = TP / (TP+FN)
Proportion with disease who test positive.

Question 5

what is specificity?

Answer 5

Specificity = TN / (TN+FP)
Proportion without disease who test negative

Question 6

What is the positive predictive value?

Answer 6

PPV = TP / (TP+FP)
Proportion with positive test who have disease

Question 7

What is the ROC curve?

Answer 7

• sensitivity % (TP) on y axis plotted against 100-specificity % (FP) on x axis
  
  • useful test should have plots above 45 degrees
  • improved by combining different tests together

Question 8

what test do we use for this

“if someone has the disease, what is the probability that the test will be positive?”

Answer 8

sensitivity

Question 9

what is the test for this:

“if someone does not have the disease, what is the probability that the test will be negative?”

Answer 9

specificity

Question 10

is screening a form of primary or secondary prevention?

Answer 10

secondary

Question 10

what is the test for this:

“if someone has a positive test, what is the probability that they will have the disease?”

Answer 10

positive predictive value

Question 11

what is the difference to primary and secondary prevention?

Answer 11

• primary prevention: aiming to delay disease onset in healthy people
• secondary intervention: aims to reduce disease’s impact (AFTER DISEASE ONSET) - FOR EXAMPLE SCREENING IS AN EXAMPLE OF SECONDARY PREVENTION AS ITS DONE AFTER DISEASE ONSET

Question 12

What are the 5 criteria for introducing a screening programme?

Answer 12

1. Condition
2. Test
3. Treatment
4. Screening programme
5. Role of UK National Screening Committee (NSC)

Question 13

What are 5 conditions for screening?

Answer 13

• Criterion 1: the condition is important in terms of frequency, impact on affected individuals
• Criterion 2: Primary prevention is not effective or feasible
• Criterion 3: There is an identifiable pre-clinical stage of disease
• Criterion 4: There is a test that can separate those with a high probability of the disease from those with a low probability
• Criterion 5: Effective treatment is available; the
  outcome of early treatment is better than later
  treatment after clinical diagnosis

Question 14

What are 4 characteristics of the screening test?

Answer 14

• The test should be simple, safe, precise and have been validated
• There should be a clear distinction between ‘normal’ and ‘abnormal’ results
• The test should be acceptable to subjects, and costs should be ‘reasonable’
• There should be an agreed policy on the further diagnostic investigation of individuals with a positive test result and on the choices available

Question 15

What is lead time bias?

Answer 15

interval between the diagnosis of a disease at screening and the usual time of diagnosis (by symptoms)

IE time between:

diagnosis by screening —> diagnosis by symptoms

Question 16

What is length bias?

Answer 16

sampling - rapidly progressive disease cause the individual to consult, but less rapidly progressing cases are likely to remain for screening detection

THEREFORE slow progressing diseases are more often found by screening and have better chances of being treated

Question 17

what is selection bias?

Answer 17

those who enter screening almost invariably are more conscious than those who decline

Question 18

what is overdiagnosis

Answer 18

bias - some lesions identified as disease in a screening programme would not have presented clinically during the individual’s life time

Question 19

what are the 4 biases associated with screening

Answer 19

lead time
length biased
selection bias
overdiagnosis bias

Question 20

what are 3 solutions to biases

Answer 20

• Randomised control trials when mortality rather than survival is used as the outcome
• Survival can be used only if there is no evidence of overdiagnosis bias and observation period is from randomisation date.
• Individual and community trials

Question 21

What are the 4 advantages of screening?

Answer 21

• Improved prognosis for true positives
• Less radical treatment required
• May be resource savings
• Reassurance for those with true negative test results

Question 22

What are the 5 disadvantages of screening?

Answer 22

• Longer period of awareness for true positives
  whose prognosis is unaltered
• Over-treatment of borderline abnormalities
• False reassurance of false negatives
• Anxiety and hazard for false positives
• Hazard of screening test to all recipients

Question 23

whats the difference between sensitivity and the positive predictive value?

Answer 23

sensitivity = proportion WITH DISEASE who test positive

PPV = proportion WITH POSITIVE TEST who have disease