Diagnosis and screening

===Lecture Details===
A Lecturer; Week 12 MED1022; 
===Lecture Content===
Diagnostic tests are done when people have a problem, screening on people before they develop a problem. Diagnosis is made when probability reaches near 100%. Diagnostic tests can give false positives when provided to a wide range of people so screening tests can be used instead, which test a population to identify early onset of disease or risk factors so outcomes can be improved. Mass screening is at whole population, selective screening at high risk groups. Screening is offered to healthy rather than unhealthy people, needs to be justified. Test may be accepted for rare condition if condition is serious, and test is good and cheap
Sensitivity = a (true positive) / (true positive + false negative) (a + d)
Validity of a diagnostic test is how well it distinguishes disease and non-diseased persons. Sensitive tests are good at telling which people really do have a disease, specific good at telling those that really don't have a disease. PPV is proportion of positives that are true. Negative predictive value is ability to pick negatives that are true.
For screening, the natural disease history should be well understood, there should be a detectable early stage, treatment at an early stage must be more beneficial than a later stage, needs to be an interval for test repetition and physical and psychological costs must be weighed against benefits.
Selection bias is to do with patients chosen having particular risk factors, lead time bias is where people seem to live longer with a disease but this is just because it is detected early, length-time bias is where there is an apparent increase in survival time due to picking up slow growing tumours.
===Readings===