Diagnosis and Screening Flashcards
What is sensitivity?
Proportion of people who have that disease that test positive (test +ve/ have disease)
What is specificity?
Proportion of people who do not have the disease who test negative (test -ve/ do not have disease).
What is the positive predictive value?
Proportion of population with a positive test who have the disease (true positive/ test +ve).
What is the negative predictive value?
Proportion of population who test negative who do not have the disease (true -ve/ test -ve)
What are the requirements before the introduction of screening according to WHO?
- Disease well defined
- Prevalence in known
- Long period between the disease detection and presentation of symptoms
- Screening test is simple and safe
- The test clearly distinguishes between those with and without the disease
- Cost-effective
- Facilities need to screen and deal with the positive results
- Path for dealing with positive results is clear
- Equity of access to screening.
What features are characteristic of diagnostic and screening tests?
Specificity and sensitivity.
Which features of a test differ with prevalence and so the population?
Negative and positive predictive values.
What is lead time bias?
Extra time during which you know you have the disease if it is diagnosed by screening rather than by clinical presentation. Because of lead time bias, survival will look longer in screened individuals even if the course of their disease is unaffected
What is length time bias?
Screening tends to diagnose a disease that is less aggressive than disease that presents clinically. Because of length time bias, some cases diagnosed by screening would never present clinically if they had not been detected by screening: overdiagnosis. E.g. cancers that do not spread rapidly.
