Lecture 25: Screening Flashcards
screening as a prevention strategy
primary: screening for alcohol intake to prevent breast cancer
secondary: screening for breast cancer
tertiary: screening for bone density following chemotherapy for breast cancer
screening flow diagram
eligible population: population at high risk of disease
testing negative –> screening test (less expensive, diagnostic test) –> re-screen after a specified time period.
testing positive –> diagnostic test (gold standard test) this is often invasive and expensive
–> if test positive will have to get an intervention
–> if test negative will re-screen
objective screening initiative
to improve health outcome (morbidity, mortality and or disability)
what is the criteria for objective screening initiative
suitable disease
suitable test
suitable treatment
suitable screening programme
suitable disease
- important public health problem
–> can be relatively common or uncommon (early detection + intervention –> better outcome) e.g. phenylketonuria (PKU) missing enzyme can be treated by a special diet. - knowledge of the natural history of disease (or relationship/strength of association of risk factors to the condition)
a. detectable early
b. increased duration of pre-clinical phase (early features of disease –> clinical diagnosis) if its long, then it gives us appropriate time to screen, identify and treat
e.g. breast cancer, cervical cancer and diabetes
suitable test
reliable- provides consistent results, safe, simple, affordable, acceptable, accuracy - the ability of a test to indicate which individuals have the disease and which do out (sensitivity and specificity)
what are the diagnostic test accuracy studies
a) tests the accuracy of the screening test
b) gold standard (the diagnostic test) is the ideal test.
i- colon cancer: colonoscopy = diagnostic test, prepare before, clinician, this type of test cannot be applied to a wide population group –> so we need a screening test
c) screening test (or a less expensive diagnostic test)
i. colon cancer: faecal occult blood test
sensitivity / specificity
true positives/all with disease * 100
true negatives/all without disease * 100
fixed characteristic of the test (will not change)
predictive value
ppv: proportion who really have the disease of all people who test positive (true positives/all who test positive)
npv: proportion who are actually free of the disease of all people who test negative. test negatives/all those who test negative
covid-19 test
screening test: pharyngeal swab with rt-pcr
very high sensitivity and specificity
predictive values;
if the prevalence is low –> higher false positive test results. this is a problem because who don’t actually have the disease are being told that they have the disease
if prevalence is high –> higher false negative test results
suitable treatment:
evidence of early treatment leading to better outcomes
effective, acceptable and accessible treatment
evidence-based policies covering who should be offered treatment and the appropriate treatment to be offered.
suitable screening programme
benefits must outweight harm
rct evidence that screen programme will: reduce mortality + increase survival time + lead time bias + length time bias
lead time bias
if the screening programme is evaluated in terms of survival time, this may give a false impression of success
length time bias
identifies 2 patients w rapidly progressive disease
identifies 5 patients with slowly progressive disease
calculating mean survival from screened patients are impression of longer average survival than occurs in the population
likely to pick up more people with slow progressing disease type
suitable screening programme p2
adequate resourcing + agreed policy for testing, diagnosis, treatment and programme management
- cost effective
- health care system must be able to support all elements of the screening pathway (every human)
- needs to reach all those who are likely to benefit from it- specific initiatives for particular population groups.
