Preventative medicine II

Question 1

What is screening?

Answer 1

Screening is a public health service in which members of a defined population, who do not necessarily perceive they are at risk of, or are already affected by a disease or its complications, are asked a question or offered a test, to identify those individuals who are more likely to be helped than harmed by further tests or treatment to reduce the risk of a disease or its complications.

Question 2

What is sensitivity with regards to a screening program?

Answer 2

The proportion of people with the disease who are identified as having it by a positive test result.

Question 3

What is specificity with regards to a screening program?

Answer 3

Is the proportion of people without the disease who are correctly re-assured by a negative test result.

Question 4

What is the positive predictive value?

Answer 4

Is the probability that a person with a positive test result actually has the disease. Reduces with reduced prevalence.

Question 5

What is the negative predictive value?

Answer 5

The probability that a person with a negative test result does not actually have the disease.

Question 6

What are the features of a screening program with high sensitivity?

Answer 6

• Maximise identification of diseased people in the screened population
• Relatively few false negatives
• But unnecessary investigations or treatments for others
• Lots of false positives

Question 7

What are the features of a screening program with high specificity?

Answer 7

• Tend to detect only people with the disease
• Relatively few false positives
• But will also miss some people who have or at risk of the disease
• Lots of false negatives

Question 8

Under which circumstances is high sensitivity desirable?

Answer 8

• Adverse consequences of missed diagnosis for the individual e.g. late treatment might be significantly worse than early
• Adverse consequences of missed diagnosis for society e.g. serious communicable disease
• Diagnosis is to be confirmed by other tests so period of anxiety is short, or correct diagnosis is given before treatment is started

Question 9

Under which circumstances is high specificity desirable?

Answer 9

• The diagnosis is associated with anxiety or stigma
• Further investigations are time-consuming, painful or expensive
• Cases are likely to be detected by other means before it is ‘too late’ for effective treatment
• Treatment, especially if painful or expensive, is to be offered without further investigations

Question 10

What is lead time bias?

Answer 10

Early diagnosis falsely appears to prolong survival.

Question 11

What is length time bias?

Answer 11

Screening over-represents less aggressive disease.

Question 12

What are the Wilson and Jungner principles for the screening of disease?

Answer 12

• The condition sought should be an important problem
• There should be an acceptable treatment for patients with recognised disease
• Facilities for diagnosis and treatment should be available
• There should be a recognised latent or early symptomatic stage
• The natural history of the condition, including its development from latent to declared disease, should be adequately understood
• There should be a suitable test or examination
• The test or examination should be acceptable to the population
• There should be agreed policy on whom to treat as patients
• The cost of case-finding (including diagnosis and subsequent treatment of patients) should be economically balanced in relation to the possible expenditure as a whole
• Case finding should be a continuous process and not a ‘once and for all’ project.