ESA 2 Population Based Screening

Question 1

What is screening?

Answer 1

A systematic attempt to detect an unrecognised condition by the application of tests, examinations or other procedures, which can be applied rapidly (and cheaply) to distinguish between apparently well persons who probably have a disease (or its precursor) and those who probably don’t.

Question 2

What is the purpose of screening?

Answer 2

To give a better outcome compared with finding something in the usual way (having symptoms and self-reporting to health services).
Simply finding something early is not the primary objective.

Question 3

What are the criteria for screening based on the condition?

Answer 3

1. An important health problem (frequency/severity) with epidemiology, incidence, prevalence and natural history understood.
2. All the cost-effective primary prevention interventions should have been implemented as far as practicable.
3. If the carriers of a mutation are identified as a result of screening, the natural history of people with this status should be understood, inc. the psychological implications.

Question 4

What are the criteria for screening based on the test?

Answer 4

1. Simple, safe, precise and validated screening test.
2. Distribution of test values in the pop must be known and an agreed cut-off level must be defined and agreed.
3. Acceptable to target pop.
4. Agreed policy on further diagnostic investigation of those who test positive and choices available to them.
5. If the test is for a particular mutation or set of genetic variants, the method for their selection and the means through which these will be kept under review in the programme should be clearly set out.

Question 5

What are the 2 types of error made by screening tests?

Answer 5

1. False positives - referring well people for further investigation
   - stress, anxiety and inconvenience
   - direct costs
   - opportunity costs
2. False negatives - fail to refer people who have early disease form
   - inappropriate reassurance
   - possibly delay presentation with symptoms

Question 6

Which 4 features define test validity?

Answer 6

1. Sensitivity (detection rate)
2. Specificity
3. Positive predictive value
4. Negative predictive values

Question 7

What is test sensitivity?

Answer 7

Probability a diseased person will test positive.

Sensitivity = true positives / (true positives + false negatives)

If sensitivity is high, test is good at correctly identifying people with disease being screened for.

Question 8

What is test specificity?

Answer 8

Probability a non-diseased person will test negative.

Specificity = true negatives / (false positives + true negatives)