Epidemiological screening Flashcards
What is epidemiological screening and how is performed?
- Investigate apparently healthy individuals to detect unrecognized disease so can prevent/delay development or improve prognosis
- If earlier detection does not offer any hope of improved outcome then screening is generally not indicated
- Divide into 2 groups (more/less likely to have disease) and investigate more likely more
What are indications for screening?
- Early diagnosis for prevention
detection of disease at an early stage leads to improved prognosis - Identify high risk individuals
where interventions will reduce that risk - Identify those posing risk to others and reduce risk
infectious disease where treatment or other control measures will improve the outcome for the individual and reduce transmission
What is a the point of a screening test and what should a screening test be?
- Identify most/least likely to have condition
- Not diagnostic necessarily
- Applied when people feel healthy
- Can have true/false positives or false/true negatives
simple, safe, acceptable, inexpensive, repeatable and valid
What are the chalanges with screening?
- Screen for low prevalence population
- So need high specificity
- So lower sensitivity
What are current screening programs in the UK?
Antenatal
- Fetal anomaly
- Infections (HIV, syphilis)
- Sickle cell disease
- Physical abnormalities
Newborn - Physical exam - Hearing test - Blood spot Adult cancer - Breast, cervical, bowel
Other
- Abnormal aortic aneurysm
- Diabetic retinopathy
Additional programs
- NHS vascular health checks
- Prostate cancer
- Chlamydia screening
Cervical cancer screening
- 35-64 y/o
- Regular screening leads to reduction in cancer and saving lives
HIV screening in pregnancy
NHS breast screening program
What is the validity of a screening test?
ability to distinguish between subjects with the condition and those without
must know definitive disease status of individuals before
What is sensitivity?
ability of the test to correctly identify people with the disease
sensitivity = a ÷ (a+c)
What is specificity?
ability of the test to correctly identify people without the disease
specificity = d ÷ (b+d)
What is positive predictive value?
likelihood that a patient with a positive test result will actually have the disease
positive predictive value = a ÷ (a+b)
What is negative predictive value?
likelihood that a patient with a negative test result will not have the disease
negative predictive value = d ÷ (c+d)
What does predictive value depend on?
sensitivity and specificity AND the prevalence of the condition in the population
What are Receiver Operator Characteristics (ROC) curves
used to determine a cut-off value for a diagnostic or screening test
graphical display of the how the proportions of true positives and false positives change for each of the possible pre-determined values.
proportion of true-positives and false-positives are calculated for possible values.
choice of cut-off value for a test is informed by the attempt to maximize sensitivity and specificity
What are approaches to screening?
mass
targeted
may be a systematic programme where people are called for screening
opportunistic programme when a person presents to the doctor for some other reason and they are offered a test
What is the disease criteria for screening?
important global health problem
detectable at early age
well understood natural diseas ehistory
long period between signs and disease
What is the test criteria for screening?
cheap reliable valid safe simple
What is the diagnosis and treatment criteria for screening?
adquate fascilities
effective, acceptable and safe
cost effective
sustainable
What does feasability depend on?
how easy it is to organise the population to attend for screening, whether the screening test is acceptable, whether facilities and resources exist to carry out the necessary diagnostic tests following screening.
What are possible biases in screening efficetivity
selection
lead time: 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: some conditions may be slower in developing to a health threatening stage, that is, they have a longer preclinical 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.
What are ethical considerations of screening?
For the individual the screening test can do harm as well as giving benefit
There may be a risk attached to the screening test or subsequent diagnostic test
A false positive result can cause unnecessary anxiety
There may be other unplanned effects of a positive test
A false negative result will give false reassurance
