Lectures 11&12 - Disease prevention part 2

Question 1

what is screening?

Answer 1

the practice of investigating apparently healthy individuals to detect unrecognised disease or its precursors so that measures can be taken to prevent or delay the development of disease or improve prognosis

Question 2

what are the 3 main purposes of screening? give examples for each

Answer 2

1) to improve prognosis (e.g. screening for breast cancer)
2) to identify presence of risk factors for a disease (e.g. screening for high blood cholesterol/high BP for CVD)
3) to identify people with infectious disease to
i) improve the outcome for the individual (e.g. screening for chlamydia)
ii) prevent ongoing transmission to others (e.g. screening health workers for hep B)

Question 3

what are the limitations of screening?

Answer 3

• may do more harm than good
• false alarms
• anxiety
• treatment of early disease which may not have become a problem

Question 4

give a clinical example of where the limitations of screening can be observed

Answer 4

• for every 50,000 breast cancer screenings, 2820 women show “abnormal” results requiring further investigation
• only 129 of these 2820 had invasive cancer
• mortality in the population was reduced but there was considerable cost associated with the further investigation required due to screening results
• considerable anxiety was also likely

Question 5

what 6 features should an ideal screening test have?

Answer 5

1) simple
2) safe
3) acceptable
4) inexpensive
5) repeatable
6) valid

Question 6

what determines the validity of a test? how can it be assessed?

Answer 6

the ability to distinguish between subjects with and without the condition (sensitivity and specificity)

it can be assessed by knowing the true disease status of the individual via a definitive test - the ‘gold standard’

Question 7

what is predictive value?

Answer 7

an additional test parameter that is useful in clinical practice

the positive predictive value (PPV) is the likelihood that a patient with a positive test result will actually have the disease

the negative predictive value (NPV) is the likelihood that a patient with a negative test result will actually not have the disease

Question 8

what is sensitivity and how is it calculated?

Answer 8

the ability of the test to correctly identify people with the disease

true positive / (true positive + false negative)

Question 9

what is specificity and how is it calculated?

Answer 9

the ability of the test to correctly identify people without the disease

true negative/(true negative + false positive)

Question 10

how is PPV calculated?

Answer 10

true positive/(true positive + false positive)

Question 11

how is NPV calculated?

Answer 11

true negative/(true negative + false negative)

Question 12

what is the predictive value of a test dependent on?

Answer 12

• sensitivity
• specificity
• prevalence of the condition in the population

Question 13

what are Receiver Operator Characteristics (ROC) curves?

Answer 13

• used to determine a cut-off value for a diagnostic/screening test
• a graphical display of how the proportions of true positives and false positives change for pre-determined values
• the choice of cut-off value for a test is informed by an attempt to maximise sensitivity (proportion of true positives) and maximise specificity (proportion of true negatives)

Question 14

what are the approaches to screening?

Answer 14

• can involve the whole population (mass) or can be targeted to group who are anticipated to have an increased prevalence
• there must either be a systematic programme (people called for screening) or opportunistic programme (person presents to the doctor for other reasons and is offered the test - e.g. chlamydia screening in young people)

Question 15

list some of the major screening programmes in the UK

Answer 15

• antenatal screening
• neonatal and childhood
• cancers
• infections
• CVD

Question 16

describe antenatal screening

Answer 16

• screening for syphilis, HIV, hep B, rubella, chromosome abnormalities, foetal growth
• some offered to all pregnant women, so offered based on risk

Question 17

describe neonatal and childhood screening

Answer 17

• newborns screened for phenylketonuria, hypothyroidism, haemoglobinopathies and sickle cell disease
• babies checked for congenital hip dislocation
• later childhood screening for hearing/development problems

Question 18

describe cancer screening

Answer 18

• systematic programmes for breast and cervical cancer in women
• screening programme for bowel cancer for all men and women aged 60-69

Question 19

describe infection screening

Answer 19

• new national opportunistic programme for chlamydia in <25yos
• people attending sexual health services are offered HIV screening
• Hep B screening is mandatory for health care workers

Question 20

descrive CVD screening

Answer 20

• abdominal aortic aneurysm screening for men aged 65
• diabetic retinopathy screening for people >12yo with diabetes
• targeted and opportunistic screening for BP, high cholesterol and diabetes in primary care

Question 21

what 3 main issues does evaluation of a potential screening programme have to consider?

Answer 21

1) feasibility
2) effectiveness
3) cost

Question 22

describe the consideration of feasibility when evaluating a potential screening programme

Answer 22

• how easy it is to organise the population to attend screening
• whether the screening test is acceptable
• whether the facilities/resources exist to carry out appropriate diagnostic tests following screening

Question 23

describe the consideration of effectiveness when evaluating a potential screening programme

Answer 23

• measuring the extent to which implementing a screening programme affects the subsequent outcomes
• can be difficult to measure due to:
• selection bias = people who participate are different to people who don’t
• lead time bias = screening identifies disease that would otherwise be identified at a later stage so improvement in the length of survival due to screening may really be die to earlier date of diagnosis
• length bias = some conditions are slower in developing to a health threatening stage so are more likely to have a more favourable prognosis

Question 24

describe the consideration of cost when evaluating a potential screening programme

Answer 24

• resources for health care are limited
• relative cost-effectiveness of a screening programme compared with other forms of health care should be considered
• there are also costs involved in subsequent diagnostic testing/treatment
• HOWEVER, without screening costs will be incurred by the treatment of more advanced stages of disease

Question 25

what are the ethical considerations for screening?

Answer 25

• screening tests can do harm as well as giving benefit
• risks attached to the screening test/subsequent diagnostic tests
• anxiety due to false positives
• false reassurance due to false negatives

Question 26

what is a cervical screening test?

Answer 26

a smear test that determines abnormal cells on the cervix

Question 27

who is invited for cervical screening?

Answer 27

all women registered with a GP:

• aged 25-49 = every 3 years
• aged 50-64 = every 5 years
• over 65 = those who haven’t been screened since 50/received abnormal tests

Question 28

what is HPV testing?

Answer 28

changes in the cells of the cervix can often be caused by human papilloma virus (HPV) so when cervical screening tests show low-grade abnormalities, samples in the UK are automatically tested for HPV