Screening Flashcards
what is screening
the practice of investigating apparently healthy individuals with the object of detecting unrecognised disease or its precursors in order that measures can be taken to prevent or delay the development of disease or improve prognosis
what is the purpose of screening
When early diagnosis of disease allows early intervention which improves prognosis. When identification of high risk individuals can help target interventions to improve prognosis. When it can identify those posing a risk to others an reduce that risk
what is a screening test
identifies those most likely to have the condition. excludes those least likely to have it. not the same as a diagnostic test (usually)
what is sensitivity
ability to pick up true positives. it = a/a+c
what is specificity
ability to exclude true negatives. it = d/b+d
what is predictive value
proportion of test result that are correct
equation for positive predictive value
a/a+b
equation for negative predictive value
d/c+d
challenges for screening tests
Generally screening low prevalence populations. Therefore need a very high specificity. This often means a lower sensitivity (ie what is the cut off for saying a test is positive or negative)
what is the appeal of screening
people would prefer not to get ill. widespread belief that early detection is better.
what are some current systematic screening programmes in the UK
antenatal - downs, Edwards syndromes. new born - hearing test, blood spot. adult cancer - breast and cervical
evaluation of screening programmes
feasibility. effectiveness. cost. ethics of screening
feasiblity
will the population attend. is the test acceptable. are facilities and resources available for screening, diagnostic follow up and treatment
effectiveness
Extent to which implementing a screening programme affects subsequent outcomes. Difficult to measure due to bias
what should an ideal screening test be
simple,safe acceptable, inexpendive
Ethics of screening
May harm as well as benefit the individual. Risks of the test. Risks of subsequent diagnostic tests. Risks of subsequent treatment. False positive result causes unnecessary anxiety. False negative result will give false reassurance
what is the validity of a test
is its ability to distinguish between subjects with the condition and those without. to asses the validity of a screening test the true disease status of the individuals must be known, usually through a definitive test which is referred to as the gold standard
what are the receiver operator characteristics (ROC) curves used for
to determine a cut-off value for a diagnostic or screening test. Y axis- sensitivity vs 1-specificty (x-asis)
Choice of a particular cut-off value is informed by the attempt to maximise sensitivity and specificity
what are the receiver operator characteristics (ROC) curves used for
to determine a cut-off value for a diagnostic or screening test. Y axis- sensitivity vs 1-specificty (x-asis)
Choice of a particular cut-off value is informed by the attempt to maximise sensitivity and specificity
what are the approaches to screening
whole population (mass) or selected groups who are anticipated to have an increased prevalence of the condition (targeted). can be a systematic programme (people are called for screening) or opportunistic (people present to doctor for some other reason and are offered test)
what are the different types of bias that influence effectiveness of screening
selection bias, lead time bias and length bias
what is selection bias
people who participate in screening programmes often dffer from those who do not.
what is lead time bias
screening identifies disease that would otherwise be identified at a later stage. Improvement in survival due to screening which is really due to the earlier date of diagnosis.
what is length bias
As some conditions may be slower in developing to a health threatening stage, they no longer have a preclinical stage. More likely to be detected at a stage that has a more favourable prognosis leading to the false conclusion that screening is beneficial in lengthening lives of those found positive.
