Flashcards in Lectures 11&12 - Disease prevention part 2 Deck (28)
what is screening?
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
what are the 3 main purposes of screening? give examples for each
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)
what are the limitations of screening?
- may do more harm than good
- false alarms
- treatment of early disease which may not have become a problem
give a clinical example of where the limitations of screening can be observed
- 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
what 6 features should an ideal screening test have?
what determines the validity of a test? how can it be assessed?
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'
what is predictive value?
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
what is sensitivity and how is it calculated?
the ability of the test to correctly identify people with the disease
true positive / (true positive + false negative)
what is specificity and how is it calculated?
the ability of the test to correctly identify people without the disease
true negative/(true negative + false positive)
how is PPV calculated?
true positive/(true positive + false positive)
how is NPV calculated?
true negative/(true negative + false negative)
what is the predictive value of a test dependent on?
- prevalence of the condition in the population
what are Receiver Operator Characteristics (ROC) curves?
- 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)
what are the approaches to screening?
- 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)
list some of the major screening programmes in the UK
- antenatal screening
- neonatal and childhood
describe antenatal screening
- screening for syphilis, HIV, hep B, rubella, chromosome abnormalities, foetal growth
- some offered to all pregnant women, so offered based on risk
describe neonatal and childhood screening
- newborns screened for phenylketonuria, hypothyroidism, haemoglobinopathies and sickle cell disease
- babies checked for congenital hip dislocation
- later childhood screening for hearing/development problems
describe cancer screening
- systematic programmes for breast and cervical cancer in women
- screening programme for bowel cancer for all men and women aged 60-69
describe infection screening
- 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
descrive CVD screening
- 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
what 3 main issues does evaluation of a potential screening programme have to consider?
describe the consideration of feasibility when evaluating a potential screening programme
- 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
describe the consideration of effectiveness when evaluating a potential screening programme
- 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
describe the consideration of cost when evaluating a potential screening programme
- 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
what are the ethical considerations for screening?
- 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
what is a cervical screening test?
a smear test that determines abnormal cells on the cervix
who is invited for cervical screening?
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