screening - lecture Flashcards
what is screening
investigating apparently healthy people to detect an unrecognised disease/ its precursors so it can be prevented/delayed/prognosis improved
take those more likely to have condition further down the healthcare pathway
not same as diagnostic test - not perfect. people have no signs/symptoms of disease
Results from screening test
false/true +ve - respect to screening test
false -ve - not benefitted from program
challenges for screening tests
screen low prevalence populations
need high specificity (exclude true -ves)
lower sensitivity
why screen
detection better
first do no harm
describe screening programs
offered to defined population
total anomaly screening
newborn screening
diabetic retinopathy annually
vascular health checks - 40-74 year olds, check for stroke, kidney disease heart disease, type 2 diabetes or dementia
prostate cancer - risk management but no screening: benefits don’t outweigh risk
chlamydia - opportunistic
screening can have major benefits
case control - see women with screening and their risk of cancer and death
reduces advanced cancer and death and risk of passing down
Breast screening program
mammography
age 50-70
3 year intervals
What is validity
How well the test measures what it’s meant to
whether it can identify thos with the disease and those without
split into specificity, sensitivity +ve and -ve predictive value
how do we confirm whether a disease is present/absent
gold standard diagnostic test
Sensitivity
ability to pick up true +ves
true +ve/all people with disease
Specificity
ability to exclude true -ve
true -ve/all people don’t have disease
Predictive value
proportion of test results that are correct
+ve predictive value: true +ve/true +ve + false +ve
-ve predictive value: true -ve/false -ve + true -ve
Mammography validity
sensitivity 80%
soecificity 91%
PPV 4%
NPV >99%
what is screening coverage
the number of people who take the test
What epidemiological information is most important in a patient’s decision to take up screening
almost everyone whi has virus has cancer
prevalence of virus high
sensitivity and specificity
screening coverage - especially within her age group
indications for screening
early diagnosis improves prognosis eg cancer
identifications of high risk individuals help target interventions eg lifestyle interventions for high Bp
identify those posing a risk at others - infection, TB
Problems with breast cancer screening
leads to over diagnosis
more people diagnosed without screening
Criteria for screening
disease - important problem, well recognised pre-clinical stage, natural history understood, long latent period
screening test - valid, simple and cheap, safe, reliable
diagnosis and treatment - facilities are adequate, effective acceptable and safe treatment available, cost effective and sustainable
Feasibility
will people attend
is the test acceptable
are there the facilities
effectiveness
does it improve outcomes
reduce mortality and morbidity
problem of interpretation due to: selection bias, lead time bias (survival appears to increase due to earlier detection), length bias (cases detected through screening are less aggressive)
Cost
expensive
implementation, provision of diagnostic and treatment services
savings by reducing later stages
Ethics
doing something to someone who is not ill
risks
false +ve = anxiety
false -ve = false reassurance
Prostate cancer
high morbidity - most common male cancer
mortality - most common male cancer cause of death
tumours slow growing
first year survival 71%
more men die with than of prostate cancer
PSA - not specific to cancer, +ve predictive value 30%, -ve predictive value high
digital rectal examination - not specific, not always acceptive
Cervical cancer screening
start age - if too early normal changes occur - overtreating. too late if older than 25
girls being vaccinated in school - not being ignored
if someone younger had symptoms then the screening test would be a waste of time and they would need to go to specialist treatment
increasing the age from 20 in 2003 has not increased the amount of cervical cancer
some countries vary but the International Agency for Research on Cancer concludes “There is minimal benefit and substantial harm in screening below age 25. Organized programmes should not include women aged less than 25 years in their target populations.” general trend for countries to increase the age. UK screening is a world leader because it is reviewed regularly
age range and intervals of vaccine - 25-49 every 3 years. age 50 and up every 5 years, changes are slower. only every 3 years because the virus grows slowly so wouldn’t detect anything else, if left longet 10% of cases would be missed
if not sexual active but have been you need to continue with screening
HPV-based screening provides 60–70% greater protection against invasive cervical carcinomas compared with cytology. Data of large-scale randomised trials support initiation of HPV-based screening from age 30 years and extension of screening intervals to at least 5 years.
