S5) Population-based Screening Flashcards
How can we detect diseases?
- Spontaneous presentation
- Opportunistic case finding
- Screening
Define diagnosis
Diagnosis is the definitive identification of a suspected disease or defect by application of tests, examinations or other procedures to definitely label people as either having a disease or not having a disease
What is screening?
Screening is a systematic attempt to detect an unrecognised condition by the application of tests, examinations, or other procedures to identify healthy people who may be at increased risk of disease or condition
Outline the screening process
What is the purpose of screening?
To give a better outcome compared with finding something in the usual way (having symptoms and self reporting to health services)
One of the criteria for implementing a screening programme is Condition.
Explain this
Condition: an important health problem (frequency/severity) with epidemiology, incidence, prevalence and natural history understood
One of the criteria for implementing a screening programme is Test.
A screening test is going to make two types of error: false positive and false negative. What are these?
- False positive – refers well people for further investigation, putting them through stress, anxiety and inconvenience
- False negative – fails to refer people who have an early form of the disease, inappropriately reasurring them
What are the four features of test validity?
- Sensitivity
- Specificity
- Positive predictive value
- Negative predictive value
What is sensitivity?
Sensitivity is the proportion of people with the disease who test positive (aka detection rate)
What is specificity?
Specificity is the proportion of the people without the disease who test negative
What is positive predictive value?
Positive predictive value is the probability that someone who has tested positive actually has the disease
What influences PPV?
Positive Predictive Value is strongly influenced by the prevalence of the disease
What is negative predictive value?
Negative predictive value is the proportion of the people who test negative and actually do not have the disease
What are the implications of false positive results?
- Offered diagnostic testing with all its anxieties and risks for an absent condition
- May lead to lower uptake of screening in future and greater risk of interval cancer
What are the implications of false negative results?
- Not offered diagnostic testing when in fact they may have benefited from it
- False reassurance – may present late with symptoms as a consequence