Screening Flashcards
What is screening?
Investigating 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 is the purpose of screening?
- Early detection can lead to improved prognosis (if this isn’t true then screening isn’t carried out)
- To identify people at increased risk of developing disease where interventions will reduce the risk
- To identify those with infectious disease where treatment or control measures will improve the outcome for the individual (e.g. chlamydia screening), or prevent ongoing transmission to others (e.g. screening food handlers)
What are the limitations of screening?
- False alarms, which cause anxiety
- More harm than good - treatment of early disease which may not have become a problem
What should the ideal screening test be?
- Simple
- safe
- acceptable
- inexpensive
- repeatable
- valid (sensitive and specific)
What is the validity of a screening test?
The ability to distinguish between subjects with the condition and those without
What is sensitivity? How do you calculate it - the formula?
The ability of the test to correctly identify people with the disease
Diseased positive/ (diseased positive+ diseased negative)
What is specificity? How do you calculate it - the formulas?
The ability of the test to correctly identify people without the disease
Non diseased negative/ (non diseased positive+ non diseased negative)
What is positive predictive value (PPV)? How do you calculate it - the formula?
The likelihood that a patient with a positive test result will actually have the disease
Diseased positive/ (diseased positive+ non diseased positive)
What is negative predictive value (NPV)? How do you calculate it - the formula?
The likelihood that a patient with a negative test result will not have the disease
Non diseased negative/ (diseased negative+ non diseased negative)
What is the predictive value of a test dependent on?
sensitivity
specificity
prevalence of the condition in the population
What are Receiver Operator Characteristics (ROC) curves?
A graphical display of how the proportions of true positives and false positives change for each of the possible pre-determined values.
It’s used to determine a cut-off value for a diagnostic or screening test.
In order to set the cut-off value for a continuous variable the proportion of true-positives and false-positives are calculated for possible values.
These proportions are sensitivity and (1 - specificity).
The choice of cut-off value for a test is informed by the attempt to maximize sensitivity and specificity.
How does the predictive value vary with prevalence?
In a very low prevalence population, often the case for screening programmes, a test with a “good” specificity (99%) will still lead to a low positive predictive value.
(sensitivity is low)
What are mass and targeted screening programmes?
Mass = involves the whole population Targeted = selected groups who are anticipated to have an increased prevalence of the condition
In either of these there may be a systematic programme where people are called for screening OR an opportunistic programme when a person is offered it by a doctor
What are the major screening programmes in the UK?
Antenatal screening
– Syphilis, HIV, hepatitis B, rubella, chromosome abnormalities, foetal growth etc.
Neonatal and childhood
– New-born babies are screened for phenylketonuria, hypothyroidism, haemoglobinopathies and sickle cell disease
– Babies are also checked for congenital hip dislocation. Routine checks in later childhood screen for problems with hearing and development.
Cancers
– There are systematic programmes for breast cancer and cervical cancer in women.
– A screening programme for bowel cancer for all men and women aged 60 – 69.
Infections
– A new national opportunistic screening programme for chlamydia in young people (under 25) is currently being rolled out across the country.
– People attending sexual health services are offered screening for HIV. Hepatitis B screening is mandatory for health care workers.
Cardiovascular disease
– Abdominal aortic aneurysm screening for men aged 65. Diabetic retinopathy screening for people aged 12 and over with diabetes.
– Targeted and opportunistic screening is carried out for blood pressure, high cholesterol, diabetes in primary care.
What are the three main issues considered when evaluating a potential screening programme?
1) Feasibility
– how easy is it to organise the population to attend screening, whether the screening test is acceptable, whether facilities and resources exist to carry out the necessary diagnostic tests following screening.
2)Effectiveness
Effectiveness is evaluated by measuring the extent to which implementing a screening programme affects the subsequent outcomes. Affected by biases:
– Selection bias
– Lead time bias exists because screening identifies disease that would otherwise be identified at a later stage. This may result in an apparent improvement in the length of survival due to screening which is really due to the earlier date of diagnosis.
– Length bias exists as some conditions may be slower in developing to a health threatening stage.This means they are more likely to be detected at that stage but they may also have a more favourable prognosis leading to the false conclusion that screening is beneficial in lengthening the lives of those found positive.
3)Cost
– Resources for health care are limited
– The relative cost-effectiveness of a screening programme compared with other forms of health care should therefore be considered.
