Week 8 Flashcards
What is screening
The investigation of asymptomatic people in order to classify them as likely or unlikely to have the disease
People who appear likely to have the disease are investigated further to arrive at a final diagnosis
Those found to have disease are treated
Screening is different from clinical practice involves a “system” or “programme” not just a test
Prerequisites for screening Wilson and Junger 1968
Condition should be important public health problem
There should be a recognised pre-symptomatic or latent phase
There should be a suitable test or examination
The test should be acceptable to the target population
The natural history of the disease should be understood
There should be an agreed policy on who to treat as patients
The cost should be economically balanced in relation to the cost of medical care as a whole
Case-funding should be continuous and not a one-off project
There should be an accepted treatment for the disease
Diseases suitable for screening
Disease should be relatively common and have severe consequences
Disease must pass through a presymtpomatic phase during which it is undiagnosed but detectable
Early treatment must offer some advantage over later treatment
Screening should have evidence of net benefit
Lead time bias
Successful screening will detect disease in its presymptomatic phase
The period between detection and death could therefore be longer simply because we have observed the process for longer without actually increasing the length of time that the patient would have survived had the disease been detected at the onset of symptoms
By bringing forward the day on which the diagnosis is made the length of time between diagnosis and death will be increased by the lead time
Such lead time bias must be accounted for when comparing survival between those screened and unscreened
Length bias
Length bias reflects the fact that disease which lend themselves to be identified by screening are more likely to be indolent and less aggressive conditions
More aggressive disease is less likely to be detected by screening because it is likely to develop fully between successive routine screening points
Survival following screen detected disease may be lengthened by the relatively less aggressive nature of the disease process
Length bias may be identified by comparing the aggressiveness of disease detected clinically between screens with that detected by screening
Sensitivity
Proportion of people with condition who test positive
True positives/ true positives + false negatives
Specificity
Proportion people without the condition who test negative
True negative/ false positives + true negatives
Positive predictive value
Proportion with positive test that have the condition
True positives/ true positives + false positives
Negative predictive value
Proportion with negative test who do not have the condition
True negatives/ false negatives + true negatives
Effect of screening on individual
Sensitivity and specificity tell us only how good the test is at picking up or excluding disease
However when we have the best possible test:
-if you test positive how likely is it you really have the disease
-if you test negative how likely is it that you really dont have the disease
For these questions we need to know the predictive values (yield from screening)
Yield is an important aspect to consider when evaluating a screening programme
Predictive values
Positive predictive value= probability that a person with a positive test truly has the disease
-if a person tests positive what is the probability that he/she has the condition
Negative predictive value= probability that a person with a negative test does not have the disease
-if a person tests negative what is probability that he/she does not have the condition
PPV and NPV determined by:
-sensitivity
-specificity
-prevalence of condition
Performance of mammography
Sensitivity: in women aged over 50 ranges from 68% to 90% with most trial and programmes achieving about 85%
-in women aged 40-49 the sensitivity is lower ranging from 62% to 76%
Specificity : ranges from 82% to 97%
Positive predictive value- for the NHS breast screening programme ranges from 6% to 8% for first screens and from 12% to 14% for subsequent screens
NHS breast screening programme (NHSBSP)
NHSBSP provides free breast screening every 3 years for all women in the UK aged 50 and over
Around 2 million women are now screened in the UK each year
Women aged between 50 and 70 are routinely invited every 3 years; women over age 70 must request an appointment
In England the screening programme is being extended to include women aged 47 to 73 years old
It is estimated that the NHSBSP saves around 1300 lives each year in the UK
The NHSBSP was established in 1988
There are over 85 breast screening units across the UK
The smallest unit invites 18000 women each year and the largest around 167000
In England the budget for the NHSBSP is about £75million this corresponds to £45 to £50 per woman screened
Ductal carcinoma in situ DCIS registrations have increased substantially since the introduction of NHSBSP because the condition is usually not palpable and mostly diagnosed by mammography
DCIS accounts from 20% of screen detected cancer
Critics have concerns that identifying DCIS is over diagnosis of breast cancer as these lesions may never progress and threaten the women’s life
Treatment is usually wide local excision but 30% result in mastectomy
NHS breast screening
About 1 in 7 14% of this called back following a screening have “cancer” of these about 20% have DCIS
Recommendation 13 uptake and coverage
High priority should be given to spreading the implementation of evidence based initiatives to increase uptake. This will require an integrated system approach and should include:
-implementing test reminders for all screening programmes
-further pilots of social media campaigns with formal evaluation and rollout if successful
-spreading good practice on physical and learning disabilities
-encouraging links with faith leaders and community groups and relevant voluntary, community and social enterprise organisations that work with the NHS at national, regional and local levels to reduce health inequalities and advance equality of opportunity
-increasing awareness of trans and gender diverse issues amongst screening health professionals
-consideration of financial incentives for providers to promote out of hours and weekend appointments
