6. Screening

Question 1

What is screening?

Answer 1

The systematic attempt to detect an unrecognised condition by 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 and those who probably don’t.

Question 2

What are the three modes of disease detection?

Answer 2

Spontaneous presentation, opportunistic case finding, screening.

Question 3

What are the criteria for screening considering the disease?

Answer 3

Must be an important health problem in numbers and severity, epidemiology and natural history well understood, early detectable stage, primary prevention must be considered where possible.

Question 4

What are the criteria for screening considering the test?

Answer 4

Simle and safe, precise and valid, acceptable to the population, distribution of test values known, agreed cut off known, policy on who to investigate further known.

Question 5

What are the criteria for screening considering the treatment?

Answer 5

Effective evidence based treatment available, early diagnosis must be advantageous, agreed policy on who to treat.

Question 6

What are the criteria for screening considering the programme?

Answer 6

Other options considered, benefits > physical and psychological harm, facilities for diagnosis and treatment.

Question 7

What are false positives in screening?

Answer 7

Well people who get referred for further investigation that can be invasive.

Question 8

What are false negatives in screening?

Answer 8

Failure to refer people who do have the disease, false reassurance.

Question 9

What is the sensitivity of a screening test?

Answer 9

The proportion of people with the disease who test positive. The detection rate, or how probable it is for a case to test positive.

Question 10

How is sensitivity of a screening test calculated?

Answer 10

True positives/(true positives + false negatives)

Question 11

What is the specificity of a screening test?

Answer 11

The proportion of people without the disease who test negative. The probability a non-case will test negative.

Question 12

How is specificity of a screening test calculated?

Answer 12

True negative/(true negatives + false positives)

Question 13

What is the positive predictive value (PPV) of a screening test?

Answer 13

The probability that someone who has tested positive actually has the disease.

Question 14

What is PPV influenced by?

Answer 14

The prevalence of the disease, if it is a high prevalence, then the PPV will be higher.

Question 15

How is PPV calculated?

Answer 15

True positives/(true positives + false positives)

Question 16

What is the negative predictive value (NPV) of a screening test?

Answer 16

The proportion of people who are test negative and who actually don’t have the disease.

Question 17

How is NPV calculated?

Answer 17

True negatives/(true negatives + false negatives)

Question 18

What are the advantages of screening?

Answer 18

Early detection may improve outcome, true negatives are reassuring.

Question 19

What are the disadvantages of screening?

Answer 19

False positives expose patients to invasive diagnostic tests, false negatives provide false reassurance and mean patients aren’t offered diagnostic tests, expensive which means money can’t be spent elsewhere.

Question 20

What is a lead time bias in terms of screening?

Answer 20

Screened patients appear to survive longer but only because the date of diagnosis is shifted earlier. They live the same amount of time but longer knowing they have the disease which could negatively impact their quality of life.

Question 21

What is length time bias in terms of screening?

Answer 21

Screening programmes are better for picking up slow-growing, unthreatening cases than more aggressive cases. These are more likely to have better outcomes anyway and may have never presented as a problem anyway.

Question 22

What is selection bias in terms of screening?

Answer 22

Those who attend screening are more likely to have had better health related behaviours in the first place anyway, healthy worker bias.

Question 23

What are some of the concerns with screening?

Answer 23

It alters the doctor-patient relationship as the patients may be healthy, the screening programmes are very complex, screening is limited so can cause harm.

Question 24

What are some of the screening programmes in the UK?

Answer 24

Abdominal aortic aneurysm, bowel cancer, breast cancer, cervical cancer, diabetic retinopathy, Down’s syndrome, foetal abnormalities, PKU, sickle cell and thalassaemia.

Question 25

What are the structural critiques of screening?

Answer 25

Victim blaming as individuals have to take responsibility but harder for some, individualising pathology rather than addressing underlying cause.

Question 26

What are the surveillance critiques of screening?

Answer 26

Increasingly subject to surveillance, wider part of social control.

Question 27

What are the social constructionist critiques of screening?

Answer 27

Health and illness can be moral and give meaning to particular social relationships.

Question 28

What is the feminist critique of screening?

Answer 28

More aimed at women than men.