Screening Flashcards
Define screening
The presumptive identification of unrecognised disease or defect by the application of tests , examinations or other procedures that can be applied rapidly to sort out apparently well persons who probably have the disease from those who do not.
What are the 5 pieces of criteria that needs to be satisfied for implementing a screening programme
1) the condition
2) the test
3) the intervention
4) the screening programme
5) implementation
CRITERIA THAT NEEDS TO BE SATISFIED: the condition
- the condition that is going to be screened for must be an important health condition that has a high prevalence and natural history. Epidemiology of this health condition should be understood.
- ALL the cost effective primary intervention procedures should have been implemented as far as practicable
- natural history of people who are carriers for a disease should be understood completely and psychological implications of finding out that they are a carrier should be considered
CRITERIA THAT NEEDS TO BE SATISFIED FOR SCREENING PROGAMME To be implemented : The test
- the test itself should be simple , safe , precise and a validated screening test
- the test should be acceptable to target population
- agreed policy on further diagnostic investigation of those that test positive and choices made available to them
CRITERIA THAT NEEDS TO BE SATISFIED FOR SCREENING TO BE IMPLEMENTED : The intervention
- patients that are tested positive need to have effective intervention with evidence that intervention at the pre-symptomatic phase leads to better outcomes compared to usual care.
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CRITERIA THAT NEEDS TO BE SATISFIED FOR SCREENING TO BE IMPLEMENTED : The screening programme
- proven effectiveness in reducing mortality or morbidity
- benefit gained by individuals should outweigh any harms for example over diagnosis , over treatment , false positives , uncertain findings
- opportunity cost of the screening programme should be economically balanced
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CRITERIA THAT NEEDS TO BE considered : implementation
- adequate staff and facilities available
- clinical management and patient outcomes should be optimised in all healthcare providers
- evidence based information available to potential participants ( informed choice)
What are the two types of errors that a screening test will generate ?
1) false positive
2) false negative
False positive
It is going to refer well people for further investigation
False negative
It is going to fail to refer people who actually do have the early form of the disease
How do we assess the effectiveness of a screening test ?( 4 ways )
1) sensitivity
2) specificity
3) positive predictive value
4) negative predictive value
Define positive predictive value
- proportion of positive tests who are cases
Eg given the result of how many people show that they will have the disease ( high risk) , what proportion of them go off to have the disease
A/ A + B x 100
Negative predictive value
Proportion of negative tests ( low risk of having disease) who are not cases in the future
D/ c+d x100
Sensitivity
The true positive rate
- the probability that a patient with a positive result has the disease
- a/ a+ c x 100
Define specificity
- true negative value
- proportion of non-cases which the test correctly detects
D / D + B x 100
What factor plays a major role on influencing positive and negative predictive value ?
If there is a greater prevalence of a disease in a population then positive predictive value increases and negative predictive value decreases
- negative predictive value increases when prevalence of disease is low
What are the difficulties faced when evaluating screening programmes ?
1) lead time bias
2) length time bias
3) selection bias
What is lead time Bias
The idea that early diagnosis falsely appears to prolong survival
- screened patients appear to survive longer because they were diagnosed earlier , however patients actually live the same length of time but live longer knowing they have the disease
What is length time bias
Screening programmes are better at picking up slowing growing , unthreatening cases than aggressive , fast growing ones
- diseases that are detectable through screening are more likely to have a favourable prognosis
- this could lead to the false conclusion that screening is beneficial in lengthening lives of those positive - but we are curing people that didn’t need curing ? They r getting treatment they didn’t need
What is selection bias
- studies of screening are often skewed by health volunteer effec
- those who have regular screening are likely to do things to protect them from disease
- randomised controlled trails can help with this bias
What does the UK national screening committee define screening as ?
The process of identifying healthy people who may have an increased chance of a disease or condition
Screening timelines : what is the first group of people that would be called in for screening ?
- women aged 25-49 would be called in for cervical screening every threee years and women aged 50-64 would be called in every 5 years
What are the advantages of screening programmes ?
- can help you make better informed decisions about your health
- can help reduce the risk
- treatment from early
- can potentially save lives
What are the limitations of screening programme s
1) not 100% accurate could lead to false positives which leads to over treatment or false negatives which means that people who are at a high risk won’t actually get the treatment they need.
2) Screening programmes may create unnecessary anxiety
3) sometimes the results from screening may lead to treatment that you didn’t even need
4) more often , screening doesn’t actually increase your life expectancy. Instead it just increases the number of years you live with a disease
