Screening Flashcards
Definition of screening
Actively identifying disease/pre disease in apparently healthy subjects who may benefit from early treatment
Definition of opportunistic screening
When someone asks their doctor for a test
This type of screening is not always checked/monitored
Definition of population screening
Entire population at risk is called to be screened
Definition of the lead time bias
Interval between the diagnosis of a disease at screening and the usual time of diagnosis by symptoms
As a result, screen detected cases appear to survive longer
Definition of length biased sampling
Rapidly progressive disease causes the individual to consult but less rapidly progressing cases are likely to remain for screening
Definition of selection bias
Those who enter screening almost invariably are more conscious than those who decline
Definition of over diagnosis bias
Signs detected and identified as disease in a screening program would not have presented clinically during the individuals lifetime
What is screening and what does it not involve
Actively identifying disease/pre disease in apparently healthy subjects
Does not involve
- testing
- case finding
- opportunistic screening
What is the difference between screening and a diagnostic test
Screening
-No symptoms of condition but disease onset has occured
Diagnostic test
-Confirm/deny suspected condition once initial testing has revealed its possibility
What is the difference between primary and secondary prevention
Is screening a form or primary or secondary prevention
Primary prevention
- Whilst you’re healthy, prevent onset of disease
- eg, cholesterol lowering drugs to prevent heart disease
Secondary prevention
- Once diseased, prevents it from getting worse, but before diagnosis
- eg screening
What are the 4 types of screening that the NHS offers
What pathologies can be tested
Cancer
- Breast
- Cervical
- Bowel
Cardiovascular
- AAA
- Diabetic retinopathy
Antenatal
- SCD and thalassemia
- Downs
- Ultrasound for congenital deformities
Newborns
- SCD, CF
- Congenital hypothyroidism, inherited metabolic diseases
- Hearing and physical tests
Describe the importance of bowel cancer screening
Most cases of bowel cancer are detected in the muscular external and lymph nodes
Decreased 5% year of survival
So by screening, more cases are detected in the mucosa where the 5% year survival is high
What are the 3 criteria for screening for a disease
Condition
- Condition affects large frequency/rare but has a large impact on individuals
- Primary prevention not possible/effective
- Identifiable preclinical stage of disease
What are the criteria for screening for a disease
Test
- Must be able to separate those with a high probability of the disease from those with a low probability
- Simple, safe, precise and validated
- Clear distinction between normal and abnormal results
- Acceptable to subjects, reasonable costs
- Agreed policy on further diagnostic investigation of individuals with a +ve result
What are the properties of a screening test, what are the 4 possible results
What are the 4 calculations you can do to test for the efficacy of the test
How does prevalence affect the calculations
TP, FP
FN, TN
Sensitivity
Specificity
PPV
NPV
-if prevalence falls, PPV falls and ratio between TP:FP falls
Describe the Receiver Operating Curve
What is the ideal ROC
If ratio of TP:FP is equal => not useful
Ideally you’d like ROC to be as close to LH corner => increased specificity and sensitivity
What is the criteria for screening for a disease
Treatment
-Effective treatment available. Outcome of early treatment better than later treatment after clinical diagnosis
What is lead time bias
Interval between the diagnosis of a disease at screening and then usual time of diagnosis by symptoms
What are the 4 types of bias that can arise in a screening test
Lead time bias
Length bias
Selection bias
Overdiagnosis bias
What is length bias
Slow growing tumors may be more often detected through screening
Rapidly progressive diseases cause the individual to consult but less rapidly progressive cases to remain for screening detection
What is selection bias
Those who enter screening are more conscious than those who decline
What is over diagnosis bias
+ve screening results does not lead to the development of disease in the individuals lifetime
What are the solutions to these biases
RCT when mortality is used as the outcome instead of survival
Survival can be used if there is evidence of over diagnosis bias and observation period is from randomization date
Individual and community trials
What are the 5 properties of a good screening programme
Evidence that the program is clinically, socially and ethically acceptable to health professionals and the public
Benefits should outweigh physical and psychological harms of programme
Opportunity cost should be balanced in relation to total expenditure on medical care
Plan for managing and monitoring the screening programme and quality
Adequate staffing and facilities fo the whole programme
