Screening Flashcards
What is screening?
testing of apparently healthy individuals to detect unrecognised disease/its precursors so measures can be taken to prevent or delay the development of disease or improve prognosis
When is screening carried out?
when detection of disease at early stage leads to improved prognosis
- breast cancer (allows treatment - surgery, chemotherapy) to reduce mortality
also used for risk factors (to identify those at increased risk that can take measures to reduce this)
- screening for high BP/cholesterol to reduce CVD risk
used to identify people with infectious disease where treatment will improve outcome (chlamydia screening) or prevent transmission (screening food handlers for salmonella or health workers for hep B)
Limitations of screening?
- more harm than good via false alarms, treating early disease that would not have been problem, anxiety
- benefits don’t always outweigh risks
- wasted funds
What is the validity of a test?
ability to distinguish between subjects with the condition and those without
DESCRIBED BY SPECIFICITY AND SENSITIVITY
How can validity of screening test be assessed?
need to know true disease status of individuals, through definitive testing known as gold standard
What is sensitivity?
ability to correctly identify those with disease
a/a+c
What is specificity?
to correctly identify those without disease
b/b+d
What is positive predictive value?
likelihood that patient with positive test result has disease
a/a+b
What is negative predictive value?
likelihood that patient with negative results doesnt have disease
d/c+d
What does predictive value depend on?
sensitivity
specificity
prevalence of condition in population
What determines cut-off value for a diagnostic or screening test?
Receiver Operator Characteristics curves (ROC)
- first proportion of true positive/false negatives determined (sensitivity and specificity)
- ROC curve displays how proportions of true positives and false positives change for each pre-determined value and cut off is determined by attempting to maximise sensitivity/specificity
How does predictive value value vary with prevalence?
low prevalence –> test with high specificity lead to low PPV
prompt confirmatory tests/follow up needed to reduce anxiety/unnecessary treatment
How is screening targeted?
whole population (mass) selected group anticipated to have increased prevalence (targeted)
may be a systematic programme where people called for screening or opportunistic programme where person offered it by doctor
Antenatal screening?
offered to all pregnant women/based on risk assessments
HIV, HEP P, rubella, foetal growth, chromosomal abnormalities
Neonatal and childhood screening?
Screen for phenylketonuria, hypothyroidism, haemoglobinopathies, SCD
For congenital hip dislocation
Routine checks for hearing/development
Cancer screening?
Systematic programme for women breast/cervical
Bowel cancer for men/women 60-69
Infection screening?
- new national opportunistic chlamydia screening in young people under 25
- sexual health service attendees offered HIV screening
- all health workers screened for hep B
For CVD?
- abdominal aortic aneurysm screening for men aged 65
- diabetic retinopathy screening for aged 12+ with diabetes
- targeted/opportunistic screening for BP, high cholesterol and diabetes in primary care
What 3 issues must be evaluated when evaluating potential screening programme?
- Feasibility - how easy to organise population for screening, if acceptable test, facilities to carry out diagnostic test post screening
- Effectiveness - how much does test influence subsequent outcomes? But there is bias.
- selection bias of those who participate in screening vs those who do not
- lead time bias
- length bias - Cost
- resources limited for healthcare and competing demands for funds
- consider cost effectiveness compared with other health care forms including subsequent diagnostic tests/treatment
What is lead time bias?
screening identifies disease that would otherwise be identified at later stage showing apparent improvement in length of survival but really due to earlier diagnosis date
What is length bias?
some conditions slower to develop to life threatening stage so more likely to be detected at that stage with more favourable prognosis leading to false conclusion that screening is beneficial in lengthening life
Why would cost still increase without screening?
treating patients in more advanced stages of disease
Ethics of screening?
- may do harm and benefit
- risk of test, diagnostic test after
- false positive cause anxiety
- other unplanned effects of positive test
- false negative give false reassurance
Ethics of screening?
- may do harm and benefit
- risk of test, diagnostic test after
- false positive cause anxiety
- ## other unplanned effects of positive test
