Diagnostic Tests & Screening?

Question 1

Define Validity?

Answer 1

Validity = truth of result, lack of bias

Question 2

What is Reliability?

Answer 2

Reliability = consistency of results
* repeatability
* reproducibility

Question 3

How do we test reliability

Answer 3

Repeat measurements improve reliability

Within-observer repeatability
- take several BP readings
- duplicate or triplicate readings in biochemical assays
- tests repeated on more than one occasion Between-observer repeatability
- X-rays or histology slides read by two observers
Nevertheless, a reliable test might not give valid results

Question 4

Clasification of a test

Answer 4

TP, true positive
FP, false positive
FN, false negative
TN, true negative

Question 5

Whats the PPV?

Answer 5

PPV = TP / (TP+FP)
Proportion with positive test who have disease
If someone has a positive test, what is the probability that they will have the disease?
POSITIVE PREDICTIVE VALUE

Question 6

Whats Specificity?

Answer 6

Specificity = TN / (TN+FP)
Proportion without disease who test negative
If someone does not have the disease, what is the probability that the test result will be negative?
SPECIFICITY

Question 7

Whats Sensitivity?

Answer 7

Sensitivity = TP / (TP+FN)
Proportion with disease who test positive
If someone has the disease, what is the probability that the test result will be positive?
SENSITIVITY

Question 8

If there are 1000 subjects, what is PPV? Prevelance? Sensitivity? Specificity?

Answer 8

Question 9

What is screening?

Answer 9

• Actively identifying disease, or pre-disease, in apparently healthy subjects who may benefit from early treatment
• Testing, case-finding and ‘opportunistic screening’ are not screening
• Population screening, an entire population at risk is called to be screened

Question 10

What is the natural history of disease?

Answer 10

Question 11

When does screening take place?

Answer 11

Question 12

Criteria for implementing a screening programme

Answer 12

• Condition
• Test
• Treatment
• Screening programme
• Role of UK National Screening Committee (NSC)

Question 13

What is Criterion 1?

Answer 13

Criterion 1: the condition is important in terms of frequency, impact on affected individuals

Question 14

What is Criterion 2?

Answer 14

Criterion 2: Primary prevention is not effective or feasible

Question 15

What is Criterion 3?

Answer 15

Criterion 3: There is an identifiable pre-clinical stage of disease

Question 16

What is Criterion 4?

Answer 16

Criterion 4: There is a test that can separate those with a high probability of the disease from those with a low probability

Question 17

What are the charectrisitics of the test?

Answer 17

• 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 18

What is Criterion 5?

Answer 18

Criterion 5: Effective treatment is available; the outcome of early treatment is better than later treatment after clinical diagnosis

Question 19

What is lead time bias?

Answer 19

Lead time bias
Screen-detected cases appear to survive longer
Lead time: interval between the diagnosis of a disease at screening and the usual time of diagnosis (by symptoms)

Question 20

What is length bias?

Answer 20

Length bias
Slow growing tumours may be more often detected through screening

Question 21

What are the Biases associated with screening?

Answer 21

1. Lead time (interval between the time of detection by screening and the time at which the disease would have been diagnosed)
2. Length-biased sampling (rapidly progressive disease cause the individual to consult, but less rapidly progressing cases are likely to remain for screening detection)
3. Selection bias (those who enter screening almost invariably are more conscious than those who decline)
4. Overdiagnosis bias (some lesions identified as disease in a screening programme would not have presented clinically during the individual’s life time)

Question 22

Solution to biases

Answer 22

• 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 23

Advantages of screening?

Answer 23

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

Question 24

Disadvantages of screening

Answer 24

• 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