Intro To Screening Flashcards
How can screening be defined?
- Public Health Service
- members of defined population who do not necessarily believe themselves to be at risk or already affected by specific condition
- asked a q/offered a test to identify individuals more likely to be helped than harmed by further tests/treatments to reduce risk of disease or its complications
What are the criteria for the condition to be screened for?
- Should be important health problem
- Epidemiology and natural history of condition should be understood
- should be detectable disease marker, latent period and Asymptomatic stage
- all cost-effective primary prevention interventions should have been implemented as far as practicable
What are the criteria for the screening test to be implemented?
- should be a simple safe precise and validated screening test
- distribution of test values in target population should be known and suitable cut off level defined
- test should be acceptable to the population
What are the criteria for the treatment in the context of the screening tests?
- effective treatment/intervention with evidence of early treatment leading to better outcomes than late treatment
- evidence from high quality RCTs that screening programme is effective in reducing morbidity/mortality
What are the criteria for implementation of a screening programme?
- Benefit should outweigh harm
- Cost economically balanced in relation to expenditure on medical care as a whole
- Adequate staffing and facilities for all stages should be available prior to commencement
- Info available explaining consequences of testing, investigation and treatment to enable informed choice
What are the different types of screening?
- Mass (PKU)
- Selective (coal miners)
- Proactive (cervical smears)
- Opportunistic (blood pressure)
Provide some examples of pre-clinical disease screening tests
- Blood pressure
- X-rays
- CT scans
- Mammography
- Cervical cytology
What is sensitivity in the context of screening tests?
- The proportion of true positives correctly identified
- Probability that it will correctly classify people with preclinical disease as positive
What is specificity in the context of screening tests?
- The proportion of true negatives correctly identified
- Probability that the test will classify people who are not diseased as negative
How do you calculate sensitivity?
TP/(TP+FN)
How do you calculate specificity?
TN/(TN+FP)
Which is more important, sensitivity or specificity?
- Usually a trade off between the two
- Good sensitivity means few false negatives (fewer mistakes)
- Good specificity means few false positives (less people worried unnecessarily)
In general, when should sensitivity be increased at the expense of specificity?
- when the disease is communicable (Ebola)
- when the disease is serious and an effective treatment is available
- when subsequent confirmatory diagnostic tests are inexpensive and low risk
What are the positive and negative predictive values?
- PPV is the likelihood that people with a positive result have the disease
- NPV is the likelihood that people with a negative result do not have the disease
How do you calculate PPV and NPV?
- PPV = TP/(TP+FP)
- NPV = TN/(TN+FN)
What are the positive and negative predictive values determined by?
- By the sensitivity and specificity of the test but also the prevalence of preclinical disease in the target population
- The lower the prevalence the lower the PPV
What does an increase in sensitivity result in?
Larger number of true positives and therefore a higher PPV
What does a decrease in specificity result in?
An increase of false positives and a large decline in PPV
The performance of a tool or measure can be assessed in terms of what two things?
- Validity: does it measure what it is supposed to measure?
- Reliability: does it give the same result on repeated application?
How often should people be screened?
- Genetic or congenital conditions = once
- Most diseases need repeat screening
- frequency derived from false negative data
What are the advantages of screening tests?
- Improved prognosis for some cases
- Less radical treatment
- Resource savings
- Reassurance for negative test results
What are the disadvantages of screening tests?
- Longer morbidity for cases where prognosis is unaltered
- Over treatment of questionable abnormalities
- Resource costs
- False reassurance
- Adverse effects (false positives)
- hazards of the test
Studies can be carried out to evaluate if the screening is effective. What are the potential biases in these evaluations?
- Volunteer bias
- Lead time bias (disease diagnosed earlier)
- Length bias (screening detects more slowly progressive disease)
- Overdiagnosis bias (would the lesion result in disease - cervical screening)
What are the different study types involved in evaluation of effectiveness of screening tests?
- Ecological
- Case control
- Cohort
- RCTs
