Screening Flashcards
What is screening?
Practice of investigating apparently healthy individuals to detect unrecognised disease or its precursors so that measures can be taken to prevent or delay the development of disease or improve prognosis
When is screening carried out?
Where the detection of disease at an early stage leads to improved prognosis
What else can screening be used for?
Risk factors and identifying people with infectious disease
What are limitations of screening?
May do more harm than good- false positives, inducing anxiety, treatment of early disease which may not have been a problem
What is the validity of a test?
Its ability to distinguish between subjects with the condition and those without
What is sensitivity?
a / (a+c) (See notes)
What is specificity?
d/ (b+d)
What is the positive predictive value?
Likelihood that a patient with a positive test will actually have the disease: a/ (a+b)
What is the negative predictive value?
Likelihood that a patient with a negative test will not have disease: d/ (c+d)
What is the predictive value dependent on?
Sensitivity, specificity, prevalence
How are cut off values determined?
Receive operator characteristics curves. ROC curve is a graphical display of the how the proportions of true positives and false positives change for each of the possible pre-determined values. The choice of cut-off value for a test is informed by the attempt to maximize sensitivity and specificity.
How does predictive value vary with prevalence?
In low prevalence population, often the case for screening programmes, a test with a “good” specificity (99%) will still lead to a low positive predictive value. Good and prompt confirmatory tests / follow-up are required in this situation to reduce unnecessary treatment or anxiety
What are the WHO criteria for evaluating screening programmes?
Feasibility, effectiveness and cost
What are 3 types of bias that affect screening?
Selection bias, lead time bias- screening identifies disease that would be identified later on- resulting in apparent improvement in length of survival
Length bias exists as 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.
Length bias: periodic screening is more likely to identify less aggressive cancers- cases identified likely to have better prognosis than those who present with symptoms
What are the current systematic screening programmes in the UK- antenatal?
Chromosome abnormalities, syphilis, HIV, Hep B, sickle cell disease and thalassemia, physical abnormalities
