Screening

Question 1

Purpose of screening

Answer 1

to spot the individuals more likely to have a disease

Question 2

Measures of screening tests

Answer 2

• For any screening test there are only two results- positive or negative
• In this case Positive means the test suggests you have the disease (not so positive)
• But for each result there are two outcomes- the test might be right or wrong

Question 3

True positive

Answer 3

positive result when the patient does actually have the disease

Question 4

False positive

Answer 4

positive result when the patient does not have the disease

Question 5

True negative

Answer 5

negative result when they don’t have the disease

Question 6

False negative

Answer 6

negative result when they do have the disease

Question 7

Sensitivity

Answer 7

Sensitivity of a test is the probability of a person with the disease obtaining a positive test result (true positive results divided by total number of people with the disease who are screened). It is a measure of how well a test picks up those with a disease. Expressed as a %

Question 8

Specificity

Answer 8

Specificity of a test is the probability of a person without the disease testing negative (true negative results divided by total number of people with the disease who are screened). It is a measure of how well a test recognises those without the disease.

Question 9

Why would number of false positives be increased?

Answer 9

If the effect of missing the disease is worse than the trauma a false result will cause, the screening service looks to improve sensitivity while specificity falls.

Question 10

Positive predictive value

Answer 10

The proportion of people with a positive test result who actually have the disease

Question 11

Negative predictive value

Answer 11

The proportion of people without the disease who are correctly excluded by the screening test

Question 12

Affects of prevalence

Answer 12

If the prevalence of a disease is high, the incidence of false positives will fall. The positive predictive value therefore increases and the negative predictive value falls.

The reverse is true for a rare disease.

Question 13

Examples of screening tests

Answer 13

• Guthrie test – for Phenylketonuria, congenital hypothyroidism, sickle cell disease, Cystic fibrosis or MCADD
• Newborn hearing screening test
• Green tie disease

Question 14

Principles

Answer 14

1. The condition should be an important problem
2. There should be an acceptable treatment
3. Facilities for diagnosis and treatment should be available
4. There should be a recognised latent or early stage
5. The natural history of the disease should be known
6. There should be a suitable test
7. The test should be acceptable to the population
8. There should be an agreed policy on whom to treat as patients
9. The cost of case finding should be economically balanced in relation to the possible expenditure as a whole
10. Case findings should be a continuous process- not once and for all

Question 15

For screening

Answer 15

• Prevent suffering
• Early identification being beneficial
• Early treatment is cheaper
• Patient satisfaction tends to be high

Question 16

Against screening

Answer 16

• Damage caused by false positives and false negatives
• Adverse effects of screening tool on healthy people
• Personal choice is compromised