L16-Cancer screening and prevention

Question 1

What is the definition of screening?

Answer 1

The investigation of asymptomatic people in order to classify them as likely or unlikely to have the disease.

Question 2

What are the WHO essential prerequisites for screening?

Answer 2

• The condition must be an important public heatlh problem
• The condition must be treatable
• There must be facilities for diagnosis and treatment
• The condition must have presymptomatic or latent phase.
• There should be a suitable test or examination
• The test should be suitable to the target population
• there should be a natural history on the condition
• There should be an agreed policy on who to treat as a patient.
• the cost should be economically balanced in relation to the cost of the medical care.
• Should be continously funded and not a one off project.

Question 3

What diseases are suitable for screening?

Answer 3

• They should be relatively common and have severe consequences
• Have a distinct preclinical phase during which it is undiagnosed but detectable
• Early treatment should have benefit over late treatment
• Should have some net benefit

Question 4

What is length time bias?

Answer 4

It is the length of time between picking up the disease and death. This will be longer in patients that are screened and picked up earlier however this may not result in an actual increase in survival.

Question 5

What is length bias?

Answer 5

It reflects the fact that diseases which lend themselves to be identified by screening tend to be less agressive conditions so it gives the bias that people survive longer when detected by screening. More aggressive conditions are likely to fully develop between routine screens so less likely to be picked up by screening.

Question 6

What is sensitivity?

Answer 6

The proportion of people identified as positives who actually have the condition
e.g. out of all those who have the condition what proportion are picked up by the test

Question 7

What is specificity?

Answer 7

The proportion of people identified as negatives who do not have the condition.
e.g. out of all those who dont have the condition what proportion are true negatives

Question 8

What is a positive predictive value?

Answer 8

The proportion of people who are true positives out of all those identified as positive.
e.g. out of all those who are given a positive result what proportion are true positives

Question 9

What is a negative predicitive value?

Answer 9

The proportion of people who are true negatives out of all those who recieved a negative result.
e.g. out of all those who are given a negative result what proportion are true negatives

Question 10

What ages does the breast screening programme include?

Answer 10

Screening every 3 years for all women aged 50 and over.
Routinely invited every 3 years when aged between 50 and 70.
In england now includes 47 to 73 years old.

Question 11

What is the critisism of the breast screening programme?

Answer 11

That 20% of picked up conditions are DCIS (ductal carcinoma in situ) which people argue may never progress into full cancer. Usually treated with wide excision or mastectomy.

Question 12

What are the main problems with the PSA test?

Answer 12

Up to Two thirds of men with elevated PSA do not have prostate cancer
Unreliable test
Substantial proportion of patients receieve unnessesary treatment