Session 6 - Screening Flashcards
Define screening
Systematic attempt to detect an unrecognised condition by use of tests, exams etc which can be applied rapidly to distinguish between persons with and without the disease
What are the 3 ways a disease can be detected?
- Spontaneous presentation
- Opportunistic case finding
- Screening
What must the disease be for a screening program to be implemented?
- Must be an important health problem
- Epidemiology and natural history must be well understood
- Must have an early detectable stage
- Cost-effective primary prevention interventions must have been considered and where possible implemented
What must the test be for the screening program to be implemented?
- Simple and Safe
- Precise and valid
- Acceptable to the population
- There must be an agreed policy on whom to investigate further
What must the treatment be for a screening program to be implemented?
- Effective evidence based treatment must be available
- Early treatment must be advantageous, must not just bring forward the date of diagnosis
What must the programme be for the screening to be implemented?
- Other options considered, e.g. improving treatment
- Benefit should outweigh physical and psychological harm
- Facilities for diagnosis and treatment
What is a false positive?
Screening programme refers well people for further investigation
What is a false negative?
Failure to refer people who do actually have early disease
What is sensitivity?
The proportion of the people with the disease who are correctly identified by screening test
Correctly Positive screening test/ (CPST+Negative screening test but has disease)
What is the specificity?
The proportion of the people without the disease who are test negative
“If I don’t have the disease, will I test negative?”
What is positive predictive value?
The probability that someone who has tested positive actually has the disease
PPV = True positives / (true + false positives)
What is negative predictive value?
Proportion of the people who are test negative who actually do not have the disease
NPV = True negatives / (false + true negatives)
What are the advantages and disadvantages for screening for disease?
Adv:
- Early detection may improve outcome
- True negatives reassure patients
Disadv:
- False positives expose patients to invasive diagnostic tests
- False negatives falsely reassure patients
- False negatives not offered diagnostic testing that could benefit
- Expensive interventions could divert money away from treatments
What is lead time bias??
- Screened patients appear to survive longer, but only because they were diagnosed earlier
- Patients live the same length of time, but longer knowing they have the disease
What is length time bias?
- Screening programmes are better at picking up slow-growing, unthreatening cases than aggressive, fast-growing ones
- Diseases that are detectable through screening are more likely to have favourable prognosis, and may indeed never have caused a problem
